iscsi: Allocate a dummy PDU for the internal nexus reset task.

When an iSCSI target session is terminated, an internal nexus reset
task is posted to abort existing tasks belonging to the session.
Previously, the ctl_io for this internal nexus reset stored a pointer
to the session in the slot that normally holds a pointer to the PDU
from the initiator that triggered the I/O request. The completion
handler then assumed that any nexus reset I/O was due to an internal
request and fetched the session pointer (instead of the PDU pointer)
from the ctl_io. However, it is possible to trigger a nexus reset via
an on-the-wire task management PDU. If such a PDU were sent to the
target, then the completion handler would incorrectly treat this
request as an internal request and treat the pointer to the received
PDU as a pointer to the session instead.

To fix, allocate a dummy PDU for the internal reset task and use an
invalid opcode to differentiate internal nexus resets from resets
requested by the initiator.

PR: 260449
Reported by: Robert Morris <rtm@lcs.mit.edu>
Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D34055