ext4: mballoc: don't replace the current preallocation group unnecessarily

In ext4_mb_check_group_pa(), the current preallocation space is
replaced with a new preallocation space when the two have the same
distance from the goal block.

This doesn't actually gain us anything, so change things so that the
function only switches to the new preallocation group if its distance
from the goal block is strictly smaller than the current preallocaiton
group's distance from the goal block.

Signed-off-by: Coly Li <bosong.ly@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>