amd64: fix tlb shootdown when all cpus are passed in the bitmap

[mjg]

Summary

Right now the routine leaves the current CPU in the map, later tripping on an assert when filling in the scoreboard:
panic: IPI scoreboard is zero, initiator 1 target 1

Instead pre-check if all CPUs are present in the map and remember that outcome for later.

Fixes 7eaea04a5bb1dc ("amd64: compare TLB shootdown target to all_cpus")

As a side note this code looks weirdly inefficient. For example the initial scoreboard loop should have a form of FOREACH macro instead of ffsing from scratch every time. That goes beyond the scope of this change, I may get around to it later.

[kib]

I like is_all introduction, but I believe that single-target or UP cases still should result in local_cb handling.

FWIW I have this staged and your review came just before I typed commit. https://kib.kiev.ua/git/gitweb.cgi?p=deviant3.git;a=commitdiff;h=686232280ac30de567d178c1f77c4b63d6253e21

[mjg]

To my reading this is preserved -- if there is only one recipient and it is curcpu this goes to local_cb as previously due to the resulting mask being empty ; for any other case it still ends up executing curcpu_cb(pmap, addr1, addr2).