btrfs: rename tree-defrag.c to defrag.c

This currently has only one helper in it, and it's for tree based
defrag.  We have the various defrag code in 3 different places, so
rename this to defrag.c.  Followup patches will move the code into this
new file.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>