Plug some ifaddr refcount leaks.

• Only take an ifaddr ref in in rt_exportinfo() if the caller explicitly requests it. Take care to release it in this case.
• Don't unconditionally take a ref in rtrequest1_fib(). rt_getifa_fib() will acquire a reference, in which case we would previously acquire two references.
• Stop taking a reference in rtinit1() before calling rtrequest1_fib(). rtrequest1_fib() will acquire a reference for the RTM_ADD case.

PR: 242746
Reviewed by: melifaro (previous version)
Tested by: ghuckriede@blackberry.com
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22912