riscv: Fix handling of interrupts during kernel page faults

We were checking the wrong status bit when deciding whether to enable
interrupts.

Reviewed by: jrtc27
Fixes: c226f193515c ("riscv: Permit spurious faults in kernel mode")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49897

(cherry picked from commit 0ca5e3dd1ad006a5251a391676e90ec4dc94df20)