afs: Disable use of the fscache I/O routines

Disable use of the fscache I/O routined by the AFS filesystem.  It's about
to transition to passing iov_iters down and fscache is about to have its
I/O path to use iov_iter, so all that needs to change.

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/158861209824.340223.1864211542341758994.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/159465768717.1376105.2229314852486665807.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/160588457929.3465195.1730097418904945578.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161118143744.1232039.2727898205333669064.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161161039077.2537118.7986870854927176905.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/161340403323.1303470.8159439948319423431.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539547167.286939.3536238932531122332.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653802797.2770958.547311814861545911.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789085806.6155.2596146255056027428.stgit@warthog.procyon.org.uk/ # v6