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>