[PATCH] md: Avoid overflow in raid0 calculation with large components.

[PATCH] md: Avoid overflow in raid0 calculation with large components.

If a raid0 has a component device larger than 4TB, and is accessed on
a 32bit machines, then as 'chunk' is unsigned lock,
   chunk << chunksize_bits
can overflow (this can be as high as the size of the device in KB).
chunk itself will not overflow (without triggering a BUG).

So change 'chunk' to be 'sector_t, and get rid of the 'BUG' as it becomes
impossible to hit.

Cc: "Jeff Zheng" <Jeff.Zheng@endace.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>