sysctl: remove __user qualifier from stack_erasing_sysctl buffer argument

sysctl: remove __user qualifier from stack_erasing_sysctl buffer argument

The buffer arg in proc handler functions have been void* (no __user
qualifier) since commit 32927393dc1c ("sysctl: pass kernel pointers to
->proc_handler"). The __user qualifier was erroneously brought back in
commit 0df8bdd5e3b3 ("stackleak: move stack_erasing sysctl to
stackleak.c"). This fixes the error by removing the __user qualifier.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510221719.3ggn070M-lkp@intel.com/
Signed-off-by: Joel Granados <joel.granados@kernel.org>