btrfs: assert on non-empty delayed iputs

I ran into an issue where there was some reference being held on an
inode that I couldn't track.  This assert wasn't triggered, but it at
least rules out we're doing something stupid.

Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>