Implement optional table entry limits for if_llatbl.

Implement counting of table entries linked on a per-table base
with an optional (if set > 0) limit of the maximum number of table
entries.

For that the public lltable_link_entry() and lltable_unlink_entry()
functions as well as the internal function pointers change from void
to having an int return type.

Given no consumer currently sets the new llt_maxentries this can be
committed on its own. The moment we make use of the table limits,
the callers of the link function must check the return value as
it can change and entries might not be added.

Adjustments for IPv6 (and possibly IPv4) will follow.

Sponsored by: Netflix (originally)
Reviewed by: melifaro
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22713