xfs_db: defang frag command

xfs_db: defang frag command

Too many people freak out about this fictitious "fragmentation
factor."  As shown in the fact, it is largely meaningless, because
the number approaches 100% extremely quickly for just a few
extents per file.

I thought about removing it altogether, but perhaps a note
about its uselessness, and a more soothing metric (avg extents
per file) might be useful.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>