generic/123, generic/128, afs: Allow for an fs that does its own perm management

generic/123, generic/128, afs: Allow for an fs that does its own perm management

The AFS filesystem has its own distributed permission management system
that's based on a per-cell user and group database used in conjunction with
ACLs.  The user is determined by the authentication token acquired from the
kaserver or Kerberos, not by the local fsuid/fsgid.  For the most part, the
uid, gid and mask on a file are ignored.

The generic/123 and generic/128 tests check that the UNIX permission bits do
what would normally be expected of them - but this fails on AFS.  Using "su"
to change the user is not effective on AFS.  Instead, "keyctl session" would
need to be used and an alternative authentication token would need to be
obtained.

Provide a "_require_unix_perm_checking" clause so that these tests can be
suppressed in cases such as AFS.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>