Ext2: remove the static function release_blocks to optimize the kernel

Because the static function 'release_blocks' is only called
when releasing blocks,it will be more simple and efficient to
call the function 'percpu_counter_add' directly.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Jan Kara <jack@suse.cz>