Introduce local_irq_enable_in_hardirq() API.  It is currently aliased to
local_irq_enable(), hence has no functional effects.

This API will be used by lockdep, but even without lockdep this will better
document places in the kernel where a hardirq context enables hardirqs.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>