amd64: Relax the assertion added in commit 4a59cbc12

We only need to ensure that interrupts are disabled when handling a
fault from iret. Otherwise it's possible to trigger the assertion
legitimately, e.g., by copying in from an invalid address.

Fixes: 4a59cbc12
Reported by: pho
Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 6cda62755612d706f30a99f70ff13ffa0f3f2422)