btrfs: remove redundant NULL check before kvfree

Fix below warnings reported by coccicheck:
./fs/btrfs/raid56.c:237:2-8: WARNING: NULL check before some freeing
functions is not needed.
Reported-by: NAbaci Robot <abaci@linux.alibaba.com>
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NYang Li <abaci-bugfix@linux.alibaba.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -233,8 +233,7 @@ int btrfs_alloc_stripe_hash_table(struct btrfs_fs_info *info)
 	}
 
 	x = cmpxchg(&info->stripe_hash_table, NULL, table);
-	if (x)
-		kvfree(x);
+	kvfree(x);
 	return 0;
 }