cifs: make compare_sids static

..nothing outside of cifsacl.c calls it. Also fix the incorrect
comment on the function. It returns 0 when they match.

Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>