GFS2: gfs2_set_acl(): Cache "no acl" as well

When removing a default acl or setting an access acl that is entirely
represented in the file mode, we end up with acl == NULL in gfs2_set_acl(). In
that case, bring gfs2 in line with other file systems and cache the NULL acl
with set_cached_acl() instead of invalidating the cache with
forget_cached_acl().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>