Cleanup unused inpcb fields/flags

[bz]

Summary

The inp_lle field to struct inpcb, along with two "valid" flags
for the rt and lle cache were added in r191129 (2009).
To my best knowledge they have never been used and route caching
has converted the inp_rt field from that commit to inp_route
rendering these fields obsolete.

Convert the pointer into a spare pointer to not change the size of
the structure anymore (and to have a spare pointer) and mark the
to fields as unused.

[glebius]

Since struct inpcb is no longer exported to the userland, the spare field can be removed.