Fix vxlan port range

[maiqueb]

Two things were wrong when attempting to create a vxlan link specifying its src port range:

• we were using native endianness (instead of big endian) to encode the port min / max ranges
• we were not updating the buffer pointer to where we were encoding the min/max values, and thus the "max" was overwriting the "min" value, causing an invalid netlink message.

Unit tests are added.

Fixes: #43
Depends-on: #42

[codecov]

Codecov Report

Merging #44 (7c56679) into main (927bdd7) will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   47.28%   47.62%   +0.33%     
==========================================
  Files          74       74              
  Lines        6349     6371      +22     
==========================================
+ Hits         3002     3034      +32     
+ Misses       3347     3337      -10     

Files Changed	Coverage Δ
src/rtnl/link/nlas/link_infos.rs	57.17% <100.00%> (+1.46%)	⬆️

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[cathay4t]

The kernel code vxlan_core.c has:

	struct ifla_vxlan_port_range ports = {
		.low =  htons(vxlan->cfg.port_min),
		.high = htons(vxlan->cfg.port_max),
	};

Approving.