nfsd4: don't create unnecessary mask acl

Any setattr of the ACL attribute, even if it sets just the basic 3-ACE
ACL exactly as it was returned from a file with only mode bits, creates
a mask entry, and it is only the mask, not group, entry that is changed
by subsequent modifications of the mode bits.

So, for example, it's surprising that GROUP@ is left without read or
write permissions after a chmod 0666:

  touch test
  chmod 0600 test
  nfs4_getfacl test
        A::OWNER@:rwatTcCy
        A::GROUP@:tcy
        A::EVERYONE@:tcy
  nfs4_getfacl test | nfs4_setfacl -S - test #
  chmod 0666 test
  nfs4_getfacl test
        A::OWNER@:rwatTcCy
        A::GROUP@:tcy
        D::GROUP@:rwa
        A::EVERYONE@:rwatcy

So, let's stop creating the unnecessary mask ACL.

A mask will still be created on non-trivial ACLs (ACLs with actual named
user and group ACEs), so the odd posix-acl behavior of chmod modifying
only the mask will still be left in that case; but that's consistent
with local behavior.
Reported-by: Soumya Koduri <skoduri@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4acl.c

@@ -542,6 +542,9 @@ posix_state_to_acl(struct posix_acl_state *state, unsigned int flags)
 	 * up setting a 3-element effective posix ACL with all
 	 * permissions zero.
 	 */
+	if (!state->users->n && !state->groups->n)
+		nace = 3;
+	else /* Note we also include a MASK ACE in this case: */
 		nace = 4 + state->users->n + state->groups->n;
 	pacl = posix_acl_alloc(nace, GFP_KERNEL);
 	if (!pacl)
@@ -586,9 +589,11 @@ posix_state_to_acl(struct posix_acl_state *state, unsigned int flags)
 		add_to_mask(state, &state->groups->aces[i].perms);
 	}
 
+	if (!state->users->n && !state->groups->n) {
 		pace++;
 		pace->e_tag = ACL_MASK;
 		low_mode_from_nfs4(state->mask.allow, &pace->e_perm, flags);
+	}
 
 	pace++;
 	pace->e_tag = ACL_OTHER;