afs: Use netfslib for directories

In the AFS ecosystem, directories are just a special type of file that is
downloaded and parsed locally.  Download is done by the same mechanism as
ordinary files and the data can be cached.  There is one important semantic
restriction on directories over files: the client must download the entire
directory in one go because, for example, the server could fabricate the
contents of the blob on the fly with each download and give a different
image each time.

So that we can cache the directory download, switch AFS directory support
over to using the netfslib single-object API, thereby allowing directory
content to be stored in the local cache.

To make this work, the following changes are made:

 (1) A directory's contents are now stored in a folio_queue chain attached
     to the afs_vnode (inode) struct rather than its associated pagecache,
     though multipage folios are still used to hold the data.  The folio
     queue is discarded when the directory inode is evicted.

     This also helps with the phasing out of ITER_XARRAY.

 (2) Various directory operations are made to use and unuse the cache
     cookie.

 (3) The content checking, content dumping and content iteration are now
     performed with a standard iov_iter iterator over the contents of the
     folio queue.

 (4) Iteration and modification must be done with the vnode's validate_lock
     held.  In conjunction with (1), this means that the iteration can be
     done without the need to lock pages or take extra refs on them, unlike
     when accessing ->i_pages.

 (5) Convert to using netfs_read_single() to read data.

 (6) Provide a ->writepages() to call netfs_writeback_single() to save the
     data to the cache according to the VM's scheduling whilst holding the
     validate_lock read-locked as (4).

 (7) Change local directory image editing functions:

     (a) Provide a function to get a specific block by number from the
     	 folio_queue as we can no longer use the i_pages xarray to locate
     	 folios by index.  This uses a cursor to remember the current
     	 position as we need to iterate through the directory contents.
     	 The block is kmapped before being returned.

     (b) Make the function in (a) extend the directory by an extra folio if
     	 we run out of space.

     (c) Raise the check of the block free space counter, for those blocks
     	 that have one, higher in the function to eliminate a call to get a
     	 block.

     (d) Remove the page unlocking and putting done during the editing
     	 loops.  This is no longer necessary as the folio_queue holds the
     	 references and the pages are no longer in the pagecache.

     (e) Mark the inode dirty and pin the cache usage till writeback at the
     	 end of a successful edit.

 (8) Don't set the large_folios flag on the inode as we do the allocation
     ourselves rather than the VM doing it automatically.

 (9) Mark the inode as being a single object that isn't uploaded to the
     server.

(10) Enable caching on directories.

(11) Only set the upload key for writeback for regular files.

Notes:

 (*) We keep the ->release_folio(), ->invalidate_folio() and
     ->migrate_folio() ops as we set the mapping pointer on the folio.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241216204124.3752367-22-dhowells@redhat.com


cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>