Fix incorrect protocol argument to RTM_DELROUTE

The following upstream kernel patch exposed a bug in RIOTest.DelRoute6:
f00f18ebb3b2 net: ipv6: check route protocol when deleting routes

This change extends the interface of iproute._Route() and specifies the
proper protocol.

Bug: 37615471
Test: net_test passes
Change-Id: I9a8db4b89bdad412cc350d42f21de95a9ed59b04