btrfs-progs: print csum values on superblock mismatch

If in btrfs_check_super() we find that the superblock has a csum
mismatch, print the wanted and found values, just as we do for metadata
in __csum_tree_block_size().

When hex-editing a btrfs image, it's useful to use btrfs check to
calculate what the new csum should be. Unfortunately at present this
only works for trees and not for the superblock, meaning you have to use
the much more wordy `btrfs inspect-internal`.

Pull-request: #985
Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: maharmstone

kernel-shared/disk-io.c

@@ -1771,8 +1771,14 @@ int btrfs_check_super(struct btrfs_super_block *sb, unsigned sbflags)
 	btrfs_csum_data(csum_type, (u8 *)sb + BTRFS_CSUM_SIZE,
 			result, BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE);
 
-	if (memcmp(result, sb->csum, csum_size)) {
-		error("superblock checksum mismatch");
+	if (memcmp(result, sb->csum, csum_size) != 0) {
+		char found[BTRFS_CSUM_STRING_LEN];
+		char wanted[BTRFS_CSUM_STRING_LEN];
+
+		btrfs_format_csum(csum_type, result, found);
+		btrfs_format_csum(csum_type, (u8 *)sb->csum, wanted);
+
+		error("superblock checksum mismatch: wanted %s found %s", wanted, found);
 		return -EIO;
 	}
 	if (btrfs_super_root_level(sb) >= BTRFS_MAX_LEVEL) {