stackleak: move stack_erasing sysctl to stackleak.c

Everyone keeps stuffing their sysctls to kernel/sysctl.c as if it
was a kitchen sink, and this creates a maintenance burden.

move stack_erasing sysctl from kernel/sysctl.c to kernel/stackleak.c.
Use register_sysctl() to register the sysctl interface.

Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>