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

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

f21afc25f9ed4 ("smp.h: Use local_irq_{save,restore}() in !SMP version of
on_each_cpu()") converted on_each_cpu() to a C function.  This required
inclusion of irqflags.h, which broke ia64 and mn10300 (at least) due to
header ordering hell.

Switch on_each_cpu() back to a macro to fix this.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Daney <david.daney@cavium.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>