btrfs: fix sizeof format specifier in btrfs_check_super_valid()

This patch fixes mips compilation warning:

fs/btrfs/disk-io.c: In function 'btrfs_check_super_valid':
fs/btrfs/disk-io.c:3927:21: warning: format '%lu' expects argument
of type 'long unsigned int', but argument 3 has type 'unsigned int' [-Wformat]
Signed-off-by: NFabian Frederick <fabf@skynet.be>
Acked-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/disk-io.c

@@ -3923,7 +3923,7 @@ static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info,
 	}
 	if (btrfs_super_sys_array_size(sb) < sizeof(struct btrfs_disk_key)
 			+ sizeof(struct btrfs_chunk)) {
-		printk(KERN_ERR "BTRFS: system chunk array too small %u < %lu\n",
+		printk(KERN_ERR "BTRFS: system chunk array too small %u < %zu\n",
 				btrfs_super_sys_array_size(sb),
 				sizeof(struct btrfs_disk_key)
 				+ sizeof(struct btrfs_chunk));