ext4: don't increment s_mb_buddies_generated in ext4_mb_release

In ext4_mb_release, we use s_mb_buddies_generated++.  Although
the output is OK, but I don't think we need this extra ++.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>