ZTS: Fix zpool_create/create-o_ashift on FreeBSD

ZTS: Fix zpool_create/create-o_ashift on FreeBSD

For some unknown reason, egrep was misbehaving with this pattern on
FreeBSD.  The command works fine run interactively from a shell, but
in the test the output of egrep is empty.

Work around the issue by using a filter in the awk script instead.

While here, add a bit of diagnostic output and other simplifications
to the awk script as well.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10023