btrfs: ref-verify: fix IS_ERR() vs NULL check in btrfs_build_ref_tree()

btrfs_extent_root()/btrfs_global_root() does not return error pointers,
it returns NULL on error.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/all/aNJfvxj0anEnk9Dm@stanley.mountain/
Fixes : ed4e6b5 ("btrfs: ref-verify: handle damaged extent root tree")
CC: stable@vger.kernel.org # 6.17+
Signed-off-by: Amit Dhingra <mechanicalamit@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: mechanicalamit

fs/btrfs/ref-verify.c

@@ -982,7 +982,7 @@ int btrfs_build_ref_tree(struct btrfs_fs_info *fs_info)
 
 	extent_root = btrfs_extent_root(fs_info, 0);
 	/* If the extent tree is damaged we cannot ignore it (IGNOREBADROOTS). */
-	if (IS_ERR(extent_root)) {
+	if (!extent_root) {
 		btrfs_warn(fs_info, "ref-verify: extent tree not available, disabling");
 		btrfs_clear_opt(fs_info->mount_opt, REF_VERIFY);
 		return 0;