ext4: remove unncessary call mb_find_buddy() in debugging code

In __mb_check_buddy(), look at the code below:
  591         fstart = -1;
  592         buddy = mb_find_buddy(e4b, 0, &max);
  593         for (i = 0; i < max; i++) {
  594                 if (!mb_test_bit(i, buddy)) {
  595                         MB_CHECK_ASSERT(i >= e4b->bd_info->bb_first_free);
  596                         if (fstart == -1) {
  597                                 fragments++;
  598                                 fstart = i;
  599                         }
  600                         continue;
  601                 }
  602                 fstart = -1;
  603                 /* check used bits only */
  604                 for (j = 0; j < e4b->bd_blkbits + 1; j++) {
  605                         buddy2 = mb_find_buddy(e4b, j, &max2);
  606                         k = i >> j;
  607                         MB_CHECK_ASSERT(k < max2);
  608                         MB_CHECK_ASSERT(mb_test_bit(k, buddy2));
  609                 }
  610         }
  611         MB_CHECK_ASSERT(!EXT4_MB_GRP_NEED_INIT(e4b->bd_info));
  612         MB_CHECK_ASSERT(e4b->bd_info->bb_fragments == fragments);
  613
  614         grp = ext4_get_group_info(sb, e4b->bd_group);
  615         buddy = mb_find_buddy(e4b, 0, &max);

On line 592, buddy is fetched by mb_find_buddy() with order 0, between
line 593 to line 615, buddy is not changed, therefore there is
no need to fetch buddy again from mb_find_buddy() with order 0 again.

We can safely remove the second mb_find_buddy() on line 615.

Signed-off-by: Coly Li <bosong.ly@taobao.com>
Cc: Alex Tomas <alex@clusterfs.com>
Cc: Theodore Tso <tytso@google.com>