When a filesystem might have hundreds of OSTs, it is useful to be able to show only the MDT stats summary without it scrolling off the screen or having to pipe it through "|grep MDT" each time.

It would be useful to add options like "lfs df --mdt" and "lfs df --ost" in lfs_df() to print only the usage on the respective MDT or OST devices in mntdf(). This should work together with existing options like "-i" to show the inode stats, or without to show the block stats, and "--pool" to print only MDTs/OSTs in that pool.

The "--ost" option is included for completeness. If both "--mdt" and "--ost" are specified then it would show both types of devices, as it does today with neither option.