Add a new light weight function to synchronize IPI queues on other CPUs by

Add a new light weight function to synchronize IPI queues on other CPUs by
broadcasting a NOP IPI to other CPUs; this is used be make sure that all
IPIs before the NOP one are processed.

Use this new function to fix a possible race between kfree() and
malloc_uninit():
kfree() may be in transitting state when malloc_uninit() is running.

Ideas-from: dillon@
Reviewed-by: dillon@