Use an RLOCK here instead of an RWLOCK - matching all the other calls
to lla_lookup().
This drastically reduces the very high lock contention when doing parallel
TCP throughput tests (> 1024 sockets) with IPv6.
MFC r260187:
lla_lookup() does modification only when LLE_CREATE is specified.
Thus we can use IF_AFDATA_RLOCK() instead of IF_AFDATA_LOCK() when doing
lla_lookup() without LLE_CREATE flag.
MFC r260217:
Add IF_AFDATA_WLOCK_ASSERT() in case lla_lookup() is called with
LLE_CREATE flag.