Enable multipage folio support for the afs filesystem.

Support has already been implemented in netfslib, fscache and cachefiles
and in most of afs, but I've waited for Matthew Wilcox's latest folio
changes.

Note that it does require a change to afs_write_begin() to return the
correct subpage.  This is a "temporary" change as we're working on
getting rid of the need for ->write_begin() and ->write_end()
completely, at least as far as network filesystems are concerned - but
it doesn't prevent afs from making use of the capability.
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: kafs-testing@auristor.com
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/lkml/2274528.1645833226@warthog.procyon.org.uk/Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>