Btrfs: convert BUG_ON to WARN_ON

These two BUG_ON()s would never be true, ensured by callers' logic.

Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>