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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30594