Do not call trapsignal() after handling usermode fault or interrupt,
when a signal is not intended to be sent.

The variable holding the signal number to send is left uninitialized,
which sometimes triggers invalid signal checks.

For NMI, a return to usermode without ast processing is done. On the
other hand, for spurious dtrace probe interrupt it is usermode which
triggered the interrupt, so handle it through userret() as any other
fault.

Reported by: Nils Beyer <nbe@renzel.net>
PR: 221151
Sponsored by: The FreeBSD Foundation
MFC after: 1 week