MFC r337558, r337560

r337558:
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

r337560:
Correct a comment. Should have been detected by ipf_nat_in() not
ipf_nat_out().