Don't check if the dataset is mounted when destroying a snapshot (libbe)

[rew]

Summary

This is for bug 236043: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236043

'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.

Test Plan

$ bectl create bootenv

should produce no errors
$ bectl destroy -o bootenv

Should I include tests for bectl into this revision?

[kevans]

Change is good; please do go ahead and include a bectl test, referencing the PR in the test's descr.

[rew]

Updating D19650: Don't check if the dataset is mounted when destroying a snapshot (libbe)

• add bectl test for PR 236043

[kevans]

Excellent, thanks! Will commit when I get a chunk of free time tomorrow night (for reference, ~22 hours from now)