lockmgr: Add missed neutering during panic

r313683 introduced new lockmgr APIs that missed the panic-time neutering
present in the rest of our locks. Correct that by adding the usual check.

Additionally, move the __lockmgr_args neutering above the assertions at the
top of the function. Drop the interlock unlock because we shouldn't have
an unneutered interlock either. No point trying to unlock it.

PR: 227749
Reported by: jtl
Sponsored by: Dell EMC Isilon