nextboot: Permit ZFS boot filesystems mounted at the pool's root

This restores nextboot -k on ZFS setups where /boot is on the root
dataset of a pool.

Reviewed by: jrtc27, glebius
Fixes: 0c3ade2cf13d nextboot: fix nextboot -k on ZFS
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D45306