btrfs: convert ASSERT(0) with handled errors to DEBUG_WARN()

The use of ASSERT(0) is maybe useful for some cases but more like a
notice for developers. Assertions can be compiled in independently so
convert it to a debugging helper.

The difference is that it's just a warning and will not end up in BUG().
The converted cases are in connection with proper error handling.

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