b=24218 fix contention on ksock_tx_t

b=24218 fix contention on ksock_tx_t

i=isaac

If a connection is closed before ksocknal_transmit() returns to
ksocknal_process_transmit(), then nobody has refcount on conn::ksnc_sock
and all pending ZC requests will be finalized by
ksocknal_connsock_decref->ksocknal_finalize_zcreq, ksocknal_finalize_zcreq
will mark not-acked ZC request as error by setting tx::tx_reside = -1.
This is race because ksocknal_process_transmit() will check tx::tx_resid
right after calling ksocknal_transmit(), and it can get
tx->tx_resid != 0 and rc == 0 then hit later LASSERT(rc < 0).