Embed dst sockaddr into rtentry and remove rte packet counter

Currently each rtentry has dst&gateway allocated separately from another zone,
bloating cache accesses.

Current 'struct rtentry' has 12 "mandatory" radix pointers in the beginning,
leaving 4 usable pointers/32 bytes in the first 2 cache lines (amd64).
Fields needed for the datapath are destination sockaddr and rt_nhop.

So far it doesn't look like there is other routable addressing protocol other
than IPv4/IPv6/MPLS, which uses keys longer than 20 bytes.
With that in mind, embed dst into struct rtentry, making the first 24 bytes
of rtentry within 128 bytes. That is enough to make IPv6 address within first
128 bytes.

It is still pretty easy to add code for supporting separately-allocated dst,
however it doesn't make a lot of sense in having such code without a use case.

As rS359823 moved the gateway to the nexthop structure, the dst embedding change
removes the need for any additional allocations done by rt_setgate().

Lastly, as a part of cleanup, remove counter(9) allocation code, as this field
is not used in packet processing anymore.

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D24669