[SCSI] iscsi_tcp regression: remove bogus warn on in write path

An empty r2tqueue is a valid state. It just means that we have
processed all that there was to do. This patch removes the WARN_ON
that was added when the kfifo changes were merged.

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