mwan3: prefer default route source address

[dhrm1k]

Prefer the src address from the interface default route before falling back
to the interface address from netifd.

This fixes the diagnostics/tracking case where an interface has one address,
but the default route explicitly selects a different source address.

Fixes: #29055

Package Details

Maintainer: @feckert

Description:
Fix mwan3_get_src_ip() so it uses the default route's preferred source
address when one is present.

---

Run Testing Details

• OpenWrt Version: 25.12.2
• OpenWrt Target/Subtarget: x86/64
• OpenWrt Device: libvirt OpenWrt VM

Tested the source address selection on an OpenWrt VM with mocked route and
netifd values.

With a default route containing src 203.0.113.50, mwan3_get_src_ip() now
returns:

route_src_case=203.0.113.50

When the default route does not contain src, it still falls back to the
interface address:

fallback_case=100.64.1.2

Local checks:

busybox ash -n feeds/packages/net/mwan3/files/lib/mwan3/common.sh
git -C feeds/packages diff --check
make package/feeds/packages/mwan3/check V=s

---

Formalities

• I have reviewed the CONTRIBUTING.md file for detailed contributing guidelines.

If your PR contains a patch:

• Not applicable, this PR does not add or modify an upstream source patch.

[feckert]

LGTM See my comments

[feckert]

This is a feature change. So we have to bump the PKG_VERSION by one and not the PKG_RELEASE.

[dhrm1k]

Ok, I will do that.