ztest: Fix ASSERT in ztest_objset_destroy_cb()

[behlendorf]

Motivation and Context

Resolves a reasonably frequent ztest failure observed which can
result in incorrect CI failures.

Description

The dsl_destroy_snapshot() call in ztest_objset_destroy_cb() may
encounter a runtime error when the pool is out of space. This is
similar to the error handling for the dsl_destroy_head() case,
but since dsl_destroy_snapshot() is implemented as a channel
program ECHRNG is returned instead of ENOSPC. ECHRNG may also
be returned instead of EBUSY if there is a hold on the snapshot.

How Has This Been Tested?

Locally reproduced and instrumented zcp_eval() to log the
error returned by dsl_sync_task_sig() before it was overwritten
with the generic ECHRNG.

I've left zloop running with the extra instrumentation to verify
these are the only cases observed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.