md/raid0: fix bug with chunksize not a power of 2.

md/raid0: fix bug with chunksize not a power of 2.

Since commit b2d231cab855222d6d684a13ed2dd1c534fa4de2
in v3.14-rc1 RAID0 has performed incorrect calculations
when the chunksize is not a power of 2.

This happens because "sector_div()" modifies its first argument, but
this wasn't taken into account in the patch.

So restore that first arg before re-using the variable.

Reported-by: Joe Landman <joe.landman@gmail.com>
Reported-by: Dave Chinner <david@fromorbit.com>
Fixes: b2d231cab855222d6d684a13ed2dd1c534fa4de2
Cc: stable@vger.kernel.org (3.14 and later).
Signed-off-by: NeilBrown <neilb@suse.de>