btrfs: return -EUCLEAN if extent item is missing when searching inline backref

At lookup_inline_extent_backref() when trying to insert an inline backref,
if we don't find the extent item we log an error and then return -EIO.
This error code is confusing because there was actually no IO error, and
this means we have some corruption, either caused by a bug or something
like a memory bitflip for example. So change the error code from -EIO to
-EUCLEAN.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -851,7 +851,7 @@ int lookup_inline_extent_backref(struct btrfs_trans_handle *trans,
 "extent item not found for insert, bytenr %llu num_bytes %llu parent %llu root_objectid %llu owner %llu offset %llu",
 			  bytenr, num_bytes, parent, root_objectid, owner,
 			  offset);
-		ret = -EIO;
+		ret = -EUCLEAN;
 		goto out;
 	}