Btrfs: eliminate a use-after-free in btrfs_balance()

Commit 5af3e8cc introduced a use-after-free at volumes.c:3139: bctl is
freed above in __cancel_balance() in all cases except for balance pause.
Fix this by moving the offending check a couple statements above, the
meaning of the check is preserved.

Reported-by: Chris Mason <chris.mason@fusionio.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>