Add ifa_try_ref() to simplify ifa handling inside epoch.

More and more code migrates from lock-based protection to the NET_EPOCH
 umbrella. It requires some logic changes, including, notably, refcount
 handling.

When we have an `ifa` pointer and we're running inside epoch we're
 guaranteed that this pointer will not be freed.
However, the following case can still happen:
 * in thread 1 we drop to 0 refcount for ifa and schedule its deletion.
 * in thread 2 we use this ifa and reference it
 * destroy callout kicks in
 * unhappy user reports bug

To address it, new `ifa_try_ref()` function is added, allowing to return
 failure when we try to reference `ifa` with 0 refcount.
Additionally, existing `ifa_ref()` is enforced with `KASSERT` to provide
 cleaner error in such scenarious.

Reviewed By: rstone, donner
Differential Revision: https://reviews.freebsd.org/D28639
MFC after:	1 week

sys/net/if.c

@@ -1857,8 +1857,18 @@ ifa_alloc(size_t size, int flags)
 void
 ifa_ref(struct ifaddr *ifa)
 {
+	u_int old;
 
-	refcount_acquire(&ifa->ifa_refcnt);
+	old = refcount_acquire(&ifa->ifa_refcnt);
+	KASSERT(old > 0, ("%s: ifa %p has 0 refs", __func__, ifa));
+}
+
+int
+ifa_try_ref(struct ifaddr *ifa)
+{
+
+	NET_EPOCH_ASSERT();
+	return (refcount_acquire_if_not_zero(&ifa->ifa_refcnt));
 }
 
 static void

sys/net/if_var.h

@@ -577,6 +577,7 @@ struct ifaddr {
 struct ifaddr *	ifa_alloc(size_t size, int flags);
 void	ifa_free(struct ifaddr *ifa);
 void	ifa_ref(struct ifaddr *ifa);
+int	ifa_try_ref(struct ifaddr *ifa);
 
 /*
  * Multicast address structure.  This is analogous to the ifaddr