copy_sighand: Handle architectures where sizeof(unsigned long) < sizeof(u64)

With the introduction of clone3 in commit 7f192e3cd3 ("fork: add
clone3") the effective bit width of clone_flags on all architectures was
increased from 32-bit to 64-bit. However, the signature of the copy_*
helper functions (e.g., copy_sighand) used by copy_process was not
adapted.

As such, they truncate the flags on any 32-bit architectures that
supports clone3 (arc, arm, csky, m68k, microblaze, mips32, openrisc,
parisc32, powerpc32, riscv32, x86-32 and xtensa).

For copy_sighand with CLONE_CLEAR_SIGHAND being an actual u64
constant, this triggers an observable bug in kernel selftest
clone3_clear_sighand:

        if (clone_flags & CLONE_CLEAR_SIGHAND)

in function copy_sighand within fork.c will always fail given:

        unsigned long /* == uint32_t */ clone_flags
        #define CLONE_CLEAR_SIGHAND 0x100000000ULL

This commit fixes the bug by always passing clone_flags to copy_sighand
via their declared u64 type, invariant of architecture-dependent integer
sizes.

Fixes: b612e5df45 ("clone3: add CLONE_CLEAR_SIGHAND")
Cc: stable@vger.kernel.org # linux-5.5+
Signed-off-by: Simon Schuster <schuster.simon@siemens-energy.com>
Link: https://lore.kernel.org/20250901-nios2-implement-clone3-v2-1-53fcf5577d57@siemens-energy.com
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

kernel/fork.c

@@ -1596,7 +1596,7 @@ static int copy_files(unsigned long clone_flags, struct task_struct *tsk,
 	return 0;
 }
 
-static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
+static int copy_sighand(u64 clone_flags, struct task_struct *tsk)
 {
 	struct sighand_struct *sig;