cifs: don't update uniqueid in cifs_fattr_to_inode

We use this value to find an inode within the hash bucket, so we can't
change this without re-hashing the inode. For now, treat this value
as immutable.

Eventually, we should probably use an inode number change on a path
based operation to indicate that the lookup cache is invalid, but that's
a bit more code to deal with.

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