btrfs: replace BUG_ON with ASSERT in walk_down_proc()

[ Upstream commit 1f9d44c0 ]

We have a couple of areas where we check to make sure the tree block is
locked before looking up or messing with references.  This is old code
so it has this as BUG_ON().  Convert this to ASSERT() for developers.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>