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

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: 7eaea04a5bb1dc86 ("amd64: compare TLB shootdown target to all_cpus")
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D28111