[AF_KEY]: no message type set

When returning a message to userspace in reply to a SADB_FLUSH or
SADB_X_SPDFLUSH message, the type was not set for the returned PFKEY
message. The patch below corrects this problem.

Signed-off-by: Jerome Borsboom <j.borsboom@erasmusmc.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>