yama: Better permission check for ptraceme

Change the permission check for yama_ptrace_ptracee to the standard
ptrace permission check, testing if the traceer has CAP_SYS_PTRACE
in the tracees user namespace.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>