The call into ifa_ifwithaddr() needs to be epoch protected; ortherwise
we'll panic on an assertion.
While here, leave a comment that the ifp was never protected and stable
(as glebius pointed out) and this needs to be fixed properly.

Discovered while working on: PR 246629
Reviewed by: glebius
MFC after: 4 days
Sponsored by: Rubicon Communications, LLC (d/b/a "Netgate")