Don't insert a flowtable entry if the lle isn't yet valid.

Some of the collisions that are occuring are due to flowtable lookups
that succeed but have an invalid lle - typically because the L2 adjacency
lookup hasn't completed.  This would lead to a follow-up insert which
would then fail (ie, collision) and the code would fall through to doing
a slow-path L2/L3 lookup in the netinet/netinet6 code.

This patch simply aborts storing a new flowtable entry if the lle isn't
yet valid.

Whilst I'm here, add a new pcpu counter for the item so the number of
failures can be tracked separately from generic "collisions."

Reviewed by:	glebius
MFC after:	10 days
Sponsored by:	Netflix, Inc.