include/linux/smp.h:on_each_cpu(): switch back to a C function

Revert commit c846ef7d ("include/linux/smp.h:on_each_cpu(): switch
back to a macro").  It turns out that the problematic linux/irqflags.h
include was fixed within ia64 and mn10300.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Daney <david.daney@cavium.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>