net: diag: slightly refactor the inet_diag_bc_audit error checks.

This simplifies the code a bit and also allows inet_diag_bc_audit
to send to userspace an error that isn't EINVAL.

[backport of net-next 627cc4add53c0470bfd118002669205d222d3a54]

Change-Id: I3afb83931e3dfb56c4c5c2f6567305981458c694
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>