bcache: add more accurate error messages in read_super()

Previous code only returns "Not a bcache superblock" for both bcache
super block offset and magic error. This patch addss more accurate error
messages,
- for super block unmatched offset:
  "Not a bcache superblock (bad offset)"
- for super block unmatched magic number:
  "Not a bcache superblock (bad magic)"
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -92,10 +92,11 @@ static const char *read_super(struct cache_sb *sb, struct block_device *bdev,
 	pr_debug("read sb version %llu, flags %llu, seq %llu, journal size %u",
 		 sb->version, sb->flags, sb->seq, sb->keys);
 
-	err = "Not a bcache superblock";
+	err = "Not a bcache superblock (bad offset)";
 	if (sb->offset != SB_SECTOR)
 		goto err;
 
+	err = "Not a bcache superblock (bad magic)";
 	if (memcmp(sb->magic, bcache_magic, 16))
 		goto err;