MFV r271511:

Use fnvlist_* to make code more readable.

Illumos issue:
    5135 zpool_find_import_cached() can use fnvlist_*

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271527 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
MFV r271512:

Illumos issue:
    5136 fix write throttle comment in dsl_pool.c

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271528 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f