scsi: libiscsi: Reject a negative task index from parse_pdu_itt

In the Linux kernel, the following vulnerability has been resolved:

scsi: libiscsi: Reject a negative task index from parse_pdu_itt

A transport that implements parse_pdu_itt hands libiscsi an index taken
from the PDU, and both lookups bound it from above only:

if (i >= session->cmds_max)

i and cmds_max are both int, so a negative index passes and
session->cmds[i] is read from before the array.

be2iscsi produces one. beiscsi_parse_pdu() assigns the raw tag,

*index = (int)itt;

and beiscsi_complete_pdu() forwards an unsolicited NOP-In from the
hardware async ring without replacing its ITT, so the value is the
target's. It also reports the session's own age rather than the one in
the tag, which leaves the age comparison in iscsi_verify_itt() with
nothing to reject.

Bound the index from below in both lookups.