i386 copyout_fast: improve detection of a fault on accessing userspace

Do not blindly account a page fault occuring on the trampoline area,
as the userspace access fault. Check that it occured exactly in the
instruction that does that.

This avoids unneeded switches of address space on faults not needing the
switch, effectively converting machine resets due to tripple faults,
into regular panics.

Reviewed by: jhb
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36302