Fix reference count leak with L2 caching.

MFC r315956

ip_forward, TCP/IPv6, and probably SCTP leaked references to L2 cache
entry because they used their own routes on the stack, not in_pcb routes.
The original model for route caching was callers that provided a route
structure to ip{,6}input() would keep the route, and this model was used
for L2 caching as well. Instead, change L2 caching to be done by default
only when using a route structure in the in_pcb; the pcb deallocation
code frees L2 as well as L3 cacches. A separate change will add route
caching to TCP/IPv6.

Another suggestion was to have the transport protocols indicate willingness
to use L2 caching, but this approach keeps the changes in the network
level

Reviewed by: ae gnn
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D10059