btrfs: move WARN_ON on unexpected error in __add_tree_block()

There's no point in having the WARN_ON(1) inside the if statement for the
unexpected error. Move it into the if statement's condition, which brings
a couple benefits:

1) It marks the branch as unlikely, hinting the compiler to generate
   better code;

2) The WARN_ON() produces a stack trace after the dumped leaf and error
   message which can hide that more important information in case we get
   a truncated dmesg/syslog.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -3200,13 +3200,12 @@ again:
 			goto again;
 		}
 	}
-	if (ret) {
+	if (WARN_ON(ret)) {
 		ASSERT(ret == 1);
 		btrfs_print_leaf(path->nodes[0]);
 		btrfs_err(fs_info,
 	     "tree block extent item (%llu) is not found in extent tree",
 		     bytenr);
-		WARN_ON(1);
 		return -EINVAL;
 	}