LU-3674 ldiskfs: fix mb_groups column names

LU-3674 ldiskfs: fix mb_groups column names

The ext4-mballoc-extra-checks.patch changes the mb_groups /proc file
to print out both the mballoc free blocks count and the group
descriptor free block count, but incorrectly modifies the column
headers.  Fix the headers to match the fields being printed.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ia7cb2348cfa7a91ed37690bdbc8438834a3ebbe5
Reviewed-on: http://review.whamcloud.com/7187
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>