Btrfs: allow mount -o remount,compress=no

Btrfs allows to turn on compression on a mounted and used filesystem
by issuing mount -o remount,compress=lzo.
This patch allows to turn compression off again
while the filesystem is mounted. As suggested by David Sterba
if the compress-force option was set, it is implicitly cleared
if compression is turned off.

Tested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Arnd Hannemann <arnd@arndnet.de>