net/smc: use announced length in sock_recvmsg()

Not every CLC proposal message needs the maximum buffer length.
Due to the MSG_WAITALL flag, it is important to use the peeked
real length when receiving the message.

Fixes: d63d271c

 ("smc: switch to sock_recvmsg()")
Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>