[SCSI] iscsi_tcp: don't fire conn error if pdu init fails

If a command's scsi cmd pdu setup fails then we can just fail
the IO to the scsi layer. If a DATA_OUT for a R2T fails then
we will want to drop the session, because it means we got a
bad request from the target (iscsi protocol error).

This patch has us propogate the error upwards so libiscsi_tcp
or libiscsi can decide what the best action is to take. It
also fixes a bug where we could try to grab the session lock
while holding it, because if iscsi_tcp drops the session in the
pdu setup callout the session lock is held when setting up the
scsi cmd pdu.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>