btrfs: defrag with compressed extents

btrfs: defrag with compressed extents

There is a long existing bug in btrfs defrag code that it will
always try to defrag compressed extents, even they are already at
max capacity.

This will not reduce the number of extents, but only waste IO/CPU.

The kernel fix is titled:

  btrfs: defrag: don't defrag extents which is already at its max capacity

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>