btrfs: let tree defrag work in SSD mode

Long time ago (2008) the defrag was automatic for new b-tree writes but
has been disabled after performance problems. There was a leftover in
tree-defrag.c that effectively stops any defragmentation on b-trees.
This is a bit unexpected and IMHO undesired. The SSD mode is an
optimization and defrag is supposed to work if the users asks for it.

Related commits:

6702ed49
Btrfs: Add run time btree defrag, and an ioctl to force btree defrag

e18e4809
Btrfs: Add mount -o ssd, which includes optimizations for seek free
storage

b3236e68
Btrfs: Leave on the tree defragger in mount -o ssd, it still helps there

9afbb0b7
Btrfs: Disable tree defrag in SSD mode

The last three commits switch the defrag+ssd off/on/off and the last one

3f157a2f
Btrfs: Online btree defragmentation fixes

misses the bits from tree-defrag.c to revert to the behaviour introduced
in e18e4809.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>