FreeBSD: Fix 12.2-STABLE after AT_BENEATH MFC

AT_BENEATH was merged to stable/12, where kern_unlinkat takes a
non-const path. DECONST the path passed to kern_unlinkat in the
case where AT_BENEATH is defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11139