fs/buffer: introduce sleeping flavors for pagecache lookups

Add __find_get_block_nonatomic() and sb_find_get_block_nonatomic()
calls for which users will be converted where safe. These versions
will take the folio lock instead of the mapping's private_lock.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://kdevops.org/ext4/v6.15-rc2.html # [0]
Link: https://lore.kernel.org/all/aAAEvcrmREWa1SKF@bombadil.infradead.org/ # [1]
Link: https://lore.kernel.org/20250418015921.132400-3-dave@stgolabs.net


Tested-by:  <kdevops@lists.linux.dev>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>