LU-11466 mdt: Skip SOM xattr update for DoM-only files

LU-11466 mdt: Skip SOM xattr update for DoM-only files

When scan the MDT image, DoM-only file can be specialized handled,
the size and blocks can be got directly from the inode on MDT, no
need SOM xattr anymore.
Thus, there is no need to store the SOM xattr for DoM-only files.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I0f871cde38fc846460dd1b6f92509dee9ea90bfc
Reviewed-on: https://review.whamcloud.com/33331
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>