[PATCH] md: allow a manual resync with md

You can trigger a 'check' with
  echo check > /sys/block/mdX/md/scan_mode
or a check-and-repair errors with
  echo repair > /sys/block/mdX/md/scan_mode

and read the current state from the same file.

Note: personalities need to know the different between 'check' and 'repair',
but don't yet.  Until they do, 'check' will be the same as 'repair' and will
just do a normal resync pass.

Signed-off-by: Neil Brown <neilb@suse.de>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>