ktime: add ms_to_ktime() and ktime_add_ms() helpers

Add two ktime helper functions that i) convert a given msec value to
a ktime structure and ii) that adds a msec value to a ktime structure.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>