stand/zfs: Refactor zfs_set_bootenv

Refactor zfs_set_bootenv to split out the lookup of spa from the
rest. zfs_set_bootenv_spa flushes the benv to the vdevs and updates the
cached benv.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D39410