route: read, store and output metric as uint32_t

[mguentner]

adding a route metric greater than 0x7fff_ffff leads to an unintended wrap when treating the metric as an unsigned int (%d) thus incorrectly rendering the metric as negative. Formatting using %u corrects the issue.

Relevant Linux patch:

https://lore.kernel.org/netdev/[email protected]/

This commit does not require a patched kernel or vice versa.