LU-9538 utils: update description of ldiskfs xattrs

Update the description of the ldiskfs xattrs on the MDT
to account for the "som" xattr added for lazy size-on-MDT.

Add a description of the xattrs on the OST.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iffd49d224b0247c0a1c719994a1355d5793ebbe5
Reviewed-on: https://review.whamcloud.com/33565
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>