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 this field and these flags 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
two fields as unused.

Reviewed by: markj, karels
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17062