squashfs: fix null-ptr-deref in squashfs_fill_super

When squashfs_read_table() returns an error or `sb->s_magic !=
SQUASHFS_MAGIC`, enters the error branch and calls
msblk->thread_ops->destroy(msblk) to destroy msblk.  However,
msblk->thread_ops has not been initialized.  Therefore, the following
problem is triggered:

==================================================================
BUG: KASAN: null-ptr-deref in squashfs_fill_super+0xe7a/0x13b0
Read of size 8 at addr 0000000000000008 by task swapper/0/1

CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.1.0-rc3-next-20221031 #367
Call Trace:
 <TASK>
 dump_stack_lvl+0x73/0x9f
 print_report+0x743/0x759
 kasan_report+0xc0/0x120
 __asan_load8+0xd3/0x140
 squashfs_fill_super+0xe7a/0x13b0
 get_tree_bdev+0x27b/0x450
 squashfs_get_tree+0x19/0x30
 vfs_get_tree+0x49/0x150
 path_mount+0xaae/0x1350
 init_mount+0xad/0x100
 do_mount_root+0xbc/0x1d0
 mount_block_root+0x173/0x316
 mount_root+0x223/0x236
 prepare_namespace+0x1eb/0x237
 kernel_init_freeable+0x528/0x576
 kernel_init+0x29/0x250
 ret_from_fork+0x1f/0x30
 </TASK>
==================================================================

To solve this issue, msblk->thread_ops is initialized immediately after
msblk is assigned a value.

Link: https://lkml.kernel.org/r/20221101073343.3961562-1-libaokun1@huawei.com
Fixes: b0645770d3c7 ("squashfs: add the mount parameter theads=<single|multi|percpu>")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Xiaoming Ni <nixiaoming@huawei.com>
Reviewed-by: Phillip Lougher <phillip@squashfs.org.uk>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/squashfs/super.c

@@ -197,6 +197,7 @@ static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
 		return -ENOMEM;
 	}
 	msblk = sb->s_fs_info;
+	msblk->thread_ops = opts->thread_ops;
 
 	msblk->panic_on_errors = (opts->errors == Opt_errors_panic);
 
@@ -231,7 +232,7 @@ static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
 			       sb->s_bdev);
 		goto failed_mount;
 	}
-	msblk->thread_ops = opts->thread_ops;
+
 	if (opts->thread_num == 0) {
 		msblk->max_thread_num = msblk->thread_ops->max_decompressors();
 	} else {