riscv/pmap.c: Handle spurious kernel page faults in critical sections

The Privileged ISA specification permits caching of invalid PTEs
12.2.1. Supervisor Memory-Management Fence Instruction), which may
result in a spurious page fault. Such faults are handled by 'pmap_fault'
which locks the kernel pmap before inspecting and possibly updating the
offending L2 entry.
Unfortunately, spurious faults may also occur when we're already holding
the kernel_pmap lock or running in a critical section, where any attempt
to grab the pmap lock will result in a kernel panic.

Fix this avoidable panic by performing a lockless lookup to determine
whether a valid kernel mapping exits and flushing appropriate TLB entry.

Differential Revision: https://reviews.freebsd.org/D56925
Reviewed by: jrtc27, mhorne, markj