wireguard: receive: reset last_under_load to zero

This is a small optimization that prevents more expensive comparisons
from happening when they are no longer necessary, by clearing the
last_under_load variable whenever we wind up in a state where we were
under load but we no longer are.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Suggested-by: Matt Dunwoodie <ncon@noconroy.net>
Signed-off-by: David S. Miller <davem@davemloft.net>