btrfs: move the file defrag code into defrag.c

This is the other big portion of defrag code that has existed in
ioctl.c.  Move it to its new home in defrag.c.
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>