ext4: fix abs() usage in ext4_mb_check_group_pa

The ext4_fsblk_t type is a long long, which should not be used
with abs(), as is done in ext4_mb_check_group_pa().

This patch modifies ext4_mb_check_group_pa() to use abs64()
instead.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>