md: Fix read balancing in RAID1 and RAID10 on drives > 2TB

read_balance uses a "unsigned long" for a sector number which
will get truncated beyond 2TB.
This will cause read-balancing to be non-optimal, and can cause
data to be read from the 'wrong' branch during a resync.  This has a
very small chance of returning wrong data.

Reported-by: Jordan Russell <jr-list-2010@quo.to>
Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>