nfs: change mkdir inode_operation to return alternate dentry if needed.

mkdir now allows a different dentry to be returned which is sometimes
relevant for nfs.

This patch changes the nfs_rpc_ops mkdir op to return a dentry, and
passes that back to the caller.

The mkdir nfs_rpc_op will return NULL if the original dentry should be
used.  This matches the mkdir inode_operation.

nfs4_do_create() is duplicated to nfs4_do_mkdir() which is changed to
handle the specifics of directories.  Consequently the current special
handling for directories is removed from nfs4_do_create()

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20250227013949.536172-6-neilb@suse.de


Signed-off-by: Christian Brauner <brauner@kernel.org>