LU-264 Integrate upstream ext4 MMP chnages

LU-264 Integrate upstream ext4 MMP chnages

Integrate the changes made to the MMP patch when it was submitted upstream
for the 2.6.40 kernel.  Mostly cosmetic changes were done (improved comments, etc),
along with moving the MMP functionality into a separate mmp.c file in order to
avoid increasing the size of super.c too much.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I6909f3e44b077d65d653994ed1ac3ee11fcacb7f
Reviewed-on: http://review.whamcloud.com/865
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>