mm/page_alloc: make bootscrub happen in idle-loop

mm/page_alloc: make bootscrub happen in idle-loop

Scrubbing RAM during boot may take a long time on machines with lots
of RAM. Add 'idle' option to bootscrub which marks all pages dirty
initially so they will eventually be scrubbed in idle-loop on every
online CPU.

It's guaranteed that the allocator will return scrubbed pages by doing
eager scrubbing during allocation (unless MEMF_no_scrub was provided).

Use the new 'idle' option as the default one.

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>