abs() function can not be used with 64 bit values, so let's switch to
abs64(). From include/linux/kernel.h:

/*
 * abs() handles unsigned and signed longs, ints, shorts and chars.
 * For all input types abs() returns a signed long.
 * abs() should not be used for 64-bit types (s64, u64, long long)
 * - use abs64() for those.
 */
Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>