btrfs: cleanup the locking in btrfs_next_old_leaf

We are carrying around this next_rw_lock from when we would do spinning
vs blocking read locks.  Now that we have the rwsem locking we can
simply use the read lock flag unconditionally and the read lock helpers.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>