btrfs: avoid null pointer dereference on fs_info when calling btrfs_crit

There are checks on fs_info in __btrfs_panic to avoid dereferencing a
null fs_info, however, there is a call to btrfs_crit that may also
dereference a null fs_info. Fix this by adding a check to see if fs_info
is null and only print the s_id if fs_info is non-null.

Detected by CoverityScan CID#401973 ("Dereference after null check")

Fixes: efe120a0 ("Btrfs: convert printk to btrfs_ and fix BTRFS prefix")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -202,7 +202,6 @@ static struct ratelimit_state printk_limits[] = {
 
 void btrfs_printk(const struct btrfs_fs_info *fs_info, const char *fmt, ...)
 {
-	struct super_block *sb = fs_info->sb;
 	char lvl[PRINTK_MAX_SINGLE_HEADER_LEN + 1] = "\0";
 	struct va_format vaf;
 	va_list args;
@@ -228,7 +227,8 @@ void btrfs_printk(const struct btrfs_fs_info *fs_info, const char *fmt, ...)
 	vaf.va = &args;
 
 	if (__ratelimit(ratelimit))
-		printk("%sBTRFS %s (device %s): %pV\n", lvl, type, sb->s_id, &vaf);
+		printk("%sBTRFS %s (device %s): %pV\n", lvl, type,
+			fs_info ? fs_info->sb->s_id : "<unknown>", &vaf);
 
 	va_end(args);
 }