btrfs-progs: defrag: add a brief warning about ref-link breakage

btrfs-progs: defrag: add a brief warning about ref-link breakage

There is a warning in btrfs-filesystem(8) saying that running 'defrag'
in Linux will almost certainly break ref-links, with much data potentially
being physically duplicated.

However, many users tend to read man pages *after* trying to run things
on their own risk and may miss this important information. This commit
adds a brief copy of this warning into the command built-in help message
where it has good chances to be spotted before user is stuck with
a crowded filesystem.

Pull-request: #73
Signed-off-by: Pavel Kretov <firegurafiku@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>