Add some more checking to the RISC-V page fault handler.

• Panic immediately if witness says we're holding non-sleepable locks. This helps ensure that we don't recurse on the pmap lock in pmap_fault_fixup().
• Panic if the kernel faults on a user address without setting an onfault handler.
• Panic if the fault occurred in a critical section or interrupt handler, like we do on other platforms.
• Fix some style issues in trap_pfault().

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18561