Avoid re-check of usermode condition.

It does not change anything in the behavior of trap_pfault(), while
eliminating obfuscation of jumping to the code which checks for the
condition reversed of the goto cause. Also avoid force initialize the
rv variable, since it is now only accessed after storing vm_fault()
return value.

Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13725