afs: Use the netfs_write_begin() helper

Make AFS use the new netfs_write_begin() helper to do the pre-reading
required before the write.  If successful, the helper returns with the
required page filled in and locked.  It may read more than just one page,
expanding the read to meet cache granularity requirements as necessary.

Note: A more advanced version of this could be made that does
generic_perform_write() for a whole cache granule.  This would make it
easier to avoid doing the download/read for the data to be overwritten.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-By: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/160588546422.3465195.1546354372589291098.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161539563244.286939.16537296241609909980.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653819291.2770958.406013201547420544.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789102743.6155.17396591236631761195.stgit@warthog.procyon.org.uk/ # v6