btrfs: ensure we don't overrun devices_info[] in __btrfs_alloc_chunk

WARN_ON isn't enough, we need to stop the loop if for any reason
we would overrun the devices_info array.

I tried to track down the connection between the length of
the alloc_devices list and the rw_devices counter but
it wasn't immediately obvious, so be defensive about it.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>