exec: Fix incorrect type for ret

In the setup_arg_pages(), ret is declared as an unsigned long.
The ret might take a negative value. Therefore, its type should
be changed to int.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20250825073609.219855-1-zhao.xichao@vivo.com


Signed-off-by: Kees Cook <kees@kernel.org>