quisce_cpus(): add special handling for PDROP

Currently passing PDROP to the quisce_cpus() function does not make sense.
Add special meaning for it, by not waiting for the idle thread to schedule.

Also avoid allocating u_int[MAXCPU] on the stack.

Reviewed by: hselasky, markj
Sponsored by: Mellanox Technologies/NVidia Networking
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30468