Better interface for hooking early initcalls

Added early initcall (pre-SMP) support, using an identical interface to
that of regular initcalls.  Functions called from do_pre_smp_initcalls()
could be converted to use this cleaner interface.

This is required by CPU hotplug, because early users have to register
notifiers before going SMP.  One such CPU hotplug user is the relay
interface with buffer-only channels, which needs to register such a
notifier, to be usable in early code.  This in turn is used by kmemtrace.

Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Cc: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>