selftests/lkdtm: Use grep -E instead of egrep

shellcheck complains that egrep is deprecated, and the grep man page
agrees. Use grep -E instead.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>