LU-2374 mkfs: Make order of --index and --ost irrelevant

LU-2374 mkfs: Make order of --index and --ost irrelevant

The order of --index and --ost should not prevent mkfs.lustre to
work. So reordered the checks which ensure that  mkfs with index is
done for ost and mdt only.

Signed-off-by: Girish Shilamkar <gshilamkar@ddn.com>
Change-Id: I228f6c9189aa85a5565393523d39e8e27e0eaddc
Reviewed-on: http://review.whamcloud.com/5942
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>