Add ifa_try_ref() to simplify ifa handling inside epoch.

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