riscv plic: Do not complete interrupts until the interrupt handler has run

We cannot complete the interrupt (i.e. write to the claims/complete register
until the interrupt handler has actually run. We don't run the interrupt
handler immediately from intr_isrc_dispatch(), we only schedule it for later
execution.

If we immediately complete it (i.e. before the interrupt handler proper has
run) the interrupt may be triggered again if the interrupt source remains set.
From RISC-V Instruction Set Manual: Volume II: Priviliged Architecture, 7.4
Interrupt Gateways:

"If a level-sensitive interrupt source deasserts the interrupt after the PLIC
core accepts the request and before the interrupt is serviced, the interrupt
request remains present in the IP bit of the PLIC core and will be serviced by
a handler, which will then have to determine that the interrupt device no
longer requires service."

In other words, we may receive interrupts twice.

Avoid that by postponing the completion until after the interrupt handler has
run.

If the interrupt is handled by a filter rather than by scheduling an interrupt
thread we must also complete the interrupt, so set up a post_filter handler
(which is the same as the post_ithread handler).

Reviewed by: mhorne
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D25531