Replace `test -b` and equivalents with `is_disk_device`, so that `-c`
is used instead on FreeBSD which has no block cache layer for devices.
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes#9795