net: remove redundant check in ip_metrics_convert()

Now ip_metrics_convert() is only called by ip_fib_metrics_init(). Before
ip_fib_metrics_init() invokes ip_metrics_convert(), it checks whether
input parameter fc_mx is NULL. Therefore, ip_metrics_convert() doesn't
need to check fc_mx.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20221104022513.168868-1-shaozhengchao@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>