FreeBSD: Initialize/destroy zp->z_lock

zp->z_lock is used in shared code for protecting projid and scantime.
We don't exercise these paths much if at all on FreeBSD, so have been
lucky enough not to have issues with the uninitialized locks so far.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #12003