cifs: on multiuser mount, set ownership to current_fsuid/current_fsgid (try #7)

commit 3aa1c8c2 made cifs_getattr set
the ownership of files to current_fsuid/current_fsgid when multiuser
mounts were in use and when mnt_uid/mnt_gid were non-zero.

It should have instead based that decision on the
CIFS_MOUNT_OVERR_UID/GID flags.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>