Btrfs: fix uninit compiler warning in cow_file_range_nocow

The extent_type variable was exposed uninit via a goto.  It should be
impossible to trigger because it is protected by a check on another
variable, but this makes sure.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/inode.c

@@ -1023,6 +1023,7 @@ next_slot:
 
 		if (found_key.offset > cur_offset) {
 			extent_end = found_key.offset;
+			extent_type = 0;
 			goto out_check;
 		}