MFC eb0b1b33: Enable multipath routing by default.

ROUTE_MPATH was added to the GENERIC kernel in r368648.

According to the plan in D27428, it was enabled with net.route.multipath sysctl set to 0.
Given enough time has passed, this change enables route multipath by default.

The goal is to ship FreeBSD 13 with multipath turned on.

Reviewed By: donner, olivier
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D28423