Btrfs: fix transaction handle leak in balance

If we fail to allocate a new data chunk, we were jumping to the error path
without release the transaction handle we got before. Fix this by always
releasing it before doing the jump.

Fixes: 2c9fe835 ("btrfs: Fix lost-data-profile caused by balance bg")
Signed-off-by: Filipe Manana <fdmanana@suse.com>