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

Commit f21afc25 ("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>
Cc: <stable@vger.kernel.org>	[3.10.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>