libbe: Fix zfs_is_mounted check w/ snapshots

'be_destroy' can destroy a boot environment (by name) or a given snapshot.
If the target to be destroyed is a dataset, check if it's mounted. We don't
want to check if the origin dataset is mounted when destroying a snapshot.

PR: 236043
Submitted by: Rob Fairbanks <rob.fx907 gmail com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19650