MFC r258631: MFV r247578

3581 spa_zio_taskq[ZIO_TYPE_FREE][ZIO_TASKQ_ISSUE]->tq_lock is piping hot
MFC r255437: MFV r247844 (illumos-gate 13975:ef6409bc370f)

Note that a different kind of cv_timedwait_hires shim is provided in
this branch because cv_timedwait_sbt is not available for better
emulation.

Sponsored by:	HybridCluster [merge]
MFC r248426: Fix typo in sysctl description

MFC slacker:	mm