Identify the return value (rval) that led to the IPv4 NAT failure
in ipf_nat_checkout() and report it in the frb_natv4out and frb_natv4in
dtrace probes.

This is currently being used to diagnose NAT failures in PR/208566. It's
rather handy so this commit makes it available for future diagnosis and
debugging efforts.

PR: 208566
MFC after: 1 week