Further refine the ExpDataSN checks for SCSI Response PDUs.

According to 11.4.8 in RFC 7143, ExpDataSN MUST be 0 if the response
code is not Command Completed, but we were requiring it to always be
the count of DataIn PDUs regardless of the response code.

In addition, at least one target (OCI Oracle iSCSI block device)
returns an ExpDataSN of 0 when returning a valid completion with an
error status (Check Condition) in response to a SCSI Inquiry. As a
workaround for this target, only warn without resetting the connection
for a 0 ExpDataSN for responses with a non-zero error status.

PR: 259152
Reported by: dch
Reviewed by: dch, mav, emaste
Fixes: 4f0f5bf99591 iscsi: Validate DataSN values in Data-In PDUs in the initiator.
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D32650

(cherry picked from commit cdbc4a074bec094dc7f1dfde0773a9417d118ffa)