Rework session termination in iSCSI target to actually wait
for any outstanding commands to be properly aborted by CTL.
Without it, in some cases (such as files backing the LUNs
stored on failing disk drives), terminating a busy session
would result in panic.

Reviewed by:	mav@ (earlier version)
Sponsored by:	The FreeBSD Foundation
Ticket:		#5255

(cherry picked from commit 69cd59fb3b0972a210fd52d029683d6af65ca421)