m68k: Fix kernel_clone_args.flags in m68k_clone()

Stan Johnson recently reported a failure from the 'dump' command:

  DUMP: Date of this level 0 dump: Fri Aug  9 23:37:15 2024
  DUMP: Dumping /dev/sda (an unlisted file system) to /dev/null
  DUMP: Label: none
  DUMP: Writing 10 Kilobyte records
  DUMP: mapping (Pass I) [regular files]
  DUMP: mapping (Pass II) [directories]
  DUMP: estimated 3595695 blocks.
  DUMP: Context save fork fails in parent 671

The dump program uses the clone syscall with the CLONE_IO flag, that is,
flags == 0x80000000. When that value is promoted from long int to u64 by
m68k_clone(), it undergoes sign-extension. The new value includes
CLONE_INTO_CGROUP so the validation in cgroup_css_set_fork() fails and
the syscall returns -EBADF. Avoid sign-extension by casting to u32.
Reported-by: Stan Johnson <userm57@yahoo.com>
Closes: https://lists.debian.org/debian-68k/2024/08/msg00000.html
Fixes: 6aabc1fa ("m68k: Implement copy_thread_tls()")
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/3463f1e5d4e95468dc9f3368f2b78ffa7b72199b.1723335149.git.fthain@linux-m68k.orgSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/kernel/process.c

@@ -117,7 +117,7 @@ asmlinkage int m68k_clone(struct pt_regs *regs)
 {
 	/* regs will be equal to current_pt_regs() */
 	struct kernel_clone_args args = {
-		.flags		= regs->d1 & ~CSIGNAL,
+		.flags		= (u32)(regs->d1) & ~CSIGNAL,
 		.pidfd		= (int __user *)regs->d3,
 		.child_tid	= (int __user *)regs->d4,
 		.parent_tid	= (int __user *)regs->d3,