[flowtable] don't insert a flowtable entry if the lle isn't yet valid

[Adrian Chadd]

Hi,

Some of the collisions stem from the flowtable entry being created and
inserted before the lle has completed - subsequent lookups will fail
the LLE_VALID check, but inserting them will immediately fail due to a
collision.

This patch:

* doesn't insert a flowtable entry until the lle is valid;
* adds a counter to netstat to log when this happens.

http://people.freebsd.org/~adrian/netflix/20140211-flowtable-no-insert-on-arp-not-done.diff

I'd like to commit this to -HEAD and backport it to -10.

Thanks!


-a