riscv trap_pfault: remove unneeded hold of the process around vm_fault() call.
This is re-appearance of the nop code removed from other arches in r287625.
Reviewed by: alc (as part of the larger patch), markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
DIfferential revision: https://reviews.freebsd.org/D21645