gfs2: Switch to may_setattr in gfs2_setattr

The permission check in gfs2_setattr is an old and outdated version of
may_setattr().  Switch to the updated version.

Fixes fstest generic/079.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>