jfs: Fix do_div precision in commit b40c2e66

In a hasty fix to replace a 64-bit division with do_div, I
unintentionally assigned the divisor to a 32-bit variable.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: Tino Reichardt <milky-kernel@mcmilk.de>