ipv4: Fix error return value in fib_convert_metrics()

The validation code modified by commit 5b5e7a0de2bb ("net: metrics:
add proper netlink validation") is organised differently in older
kernel versions.  The fib_convert_metrics() function that is modified
in the backports to 4.4 and 4.9 needs to returns an error code, not a
success flag.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>