btrfs: add "0x" prefix for unsupported optional features

The following error message lack the "0x" obviously:

  cannot mount because of unsupported optional features (4000)

Add the prefix to make it less confusing. This can happen on older
kernels that try to mount a filesystem with newer features so it makes
sense to backport to older trees.

CC: [email protected] # 4.14+
Reviewed-by: Nikolay Borisov <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: adam900710

fs/btrfs/disk-io.c

@@ -3612,7 +3612,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		~BTRFS_FEATURE_INCOMPAT_SUPP;
 	if (features) {
 		btrfs_err(fs_info,
-		    "cannot mount because of unsupported optional features (%llx)",
+		    "cannot mount because of unsupported optional features (0x%llx)",
 		    features);
 		err = -EINVAL;
 		goto fail_alloc;
@@ -3650,7 +3650,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		~BTRFS_FEATURE_COMPAT_RO_SUPP;
 	if (!sb_rdonly(sb) && features) {
 		btrfs_err(fs_info,
-	"cannot mount read-write because of unsupported optional features (%llx)",
+	"cannot mount read-write because of unsupported optional features (0x%llx)",
 		       features);
 		err = -EINVAL;
 		goto fail_alloc;