Merge r247842.

Write lock is not required for find&compare operation.
MFC r242845, r247592:

MFC r242845 (delphij):
  Illumos r13840:97fd5cdf328a:
    3145 single-copy arc
    3212 ztest: race condition between vdev_online() and spa_vdev_remove()

  Illumos r13849:3468a95b27cd:
    3258 ztest's use of file descriptors is unstable

MFC r247592 (delphij):
  Import a fix tighten assertion on SPA versions from vendor (Illumos).

  Illumos ZFS issue:
    3543 Feature flags causes assertion in spa.c to miss certain cases

Approved by:	delphij
MFC r248180:
  Take the inpcb rlock before calculating checksum, it was accidentally
  moved in r191672.