cifs: remove two cases where rc is set unnecessarily in sid_to_id

In both these cases sid_to_id unconditionally returned success, and
used the default uid/gid for the mount, so setting rc is confusing
and simply gets overwritten (set to 0) later in the function.

Addresses-Coverity: 1491672 ("Unused value")
Signed-off-by: Steve French <stfrench@microsoft.com>