[SCSI] libiscsi: don't increment cmdsn if cmd is not sent

We increment session->cmdsn at the top of iscsi_prep_scsi_cmd_pdu, but
if the prep  ecb or prep bidi or init_task calls fails then we leave the
session->cmdsn incremented. This moves the cmdsn manipulation to the end
of the function when we know it has succeeded.

It also adds a session->cmdsn--; in queuecommand for if a driver like
bnx2i tries to send a a task from that context but it fails. We do not
have to do this in the xmit thread context because that code will retry
the same task if the initial call fails.

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