target: Fix possible TFO->write_pending() sense_reason_t silent WRITE corruption

This patch fixes a possible case in transport_generic_new_cmd() where a
failure from TFO->write_pending() from a fabric module return something
other than -EAGAIN or -ENOMEM would cause a failed WRITE to silently
succeed.

Go ahead and return TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE for this
special case instead of only just making noise with WARN_ON().

(v2: Fix incorrect exception return for all cases)

Cc: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>