RAID-6 check standalone md device

RAID-6 check standalone md device

Allow RAID-6 check to be passed only the
MD device, start and length.
The three parameters are mandatory.

All necessary information is collected using
the "sysfs_read()" call.
Furthermore, if "length" is "0", then the check
is performed until the end of the array.

Some checks are done, for example if the md device
is really a RAID-6. Nevertheless I guess it is not
bullet proof...

Next patch will include the "suspend" action.
My idea is to do it "per stripe", please let me
know if you've some better options.

Signed-off-by: NeilBrown <neilb@suse.de>