fstests: add AFS support

fstests: add AFS support

Add support for the AFS filesystem.  AFS is a network filesystem and there
are a number of features it doesn't support.

 - No mkfs.  (Kind of.  An AFS volume server can be asked to create a new
   volume, but that's probably best left to AFS-specific test suites.
   Further, a volume would need to be destroyed before another of the same
   name could be created; it's not simply a matter of overwriting the old
   one as it is on a blockdev with a block-based filesystem.)

 - No fsck.  (Kind of - the server can be asked to salvage a volume, but it
   may involve taking the server offline).

 - No richacls.  AFS has its own ACL system.

 - No atimes.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>