scm: fix a typo in put_cmsg()

We need to store cmlen instead of len in cm->cmsg_len.

Fixes: 38ebcf50

 ("scm: optimize put_cmsg()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>