libzfs: FreeBSD doesn't resize partitions for you

This code can be failure prone on FreeBSD, where zfsd will pass a guid
as the vdev path to online. The guid causes zfs_resolve_shortname to
fail because it expects a path. We can just skip the whole ordeal.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #12083