net/smc: continue waiting if peer signals write_shutdown

If the peer sends a shutdown WRITE, this should not affect sending
in general, and waiting for send buffer space in particular.
Stop waiting of the local socket for send buffer space only, if peer
signals closing, but not if peer signals just shutdown WRITE.

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>