MFC r264026:

Enable a KASSERT.

Sponsored by:	The FreeBSD Foundation
MFC r264109:

We don't need TAILQ for iSCSI PDUs; STAILQ is enough.

Sponsored by:	The FreeBSD Foundation
MFC r264110:

All the iSCSI sysctls are also tunables; advertise that.

Sponsored by:	The FreeBSD Foundation
MFC r264122:

Rework the iSCSI PDU transmit code to avoid lock contention and coalesce
PDUs before sending.

Sponsored by:	The FreeBSD Foundation
MFC r264163:

Remove hack to pass STAILQ to a function and do it properly instead.

Sponsored by:	The FreeBSD Foundation
MFC r264264 by mav@:

Wakeup only one thread of added in r263978i at a time.

This slightly reduces lock congestion between threads.