[vpp] Allow routes to override Class E drop

[rustiqly]

Why I did it

SONiC VPP currently inherits upstream VPP's hard drop for 240.0.0.0/4 via FIB_SOURCE_SPECIAL. That source cannot be overridden by normal route programming, so VPP drops Class E destinations even when SONiC has an explicit/default route covering them. This breaks parity with other ASIC platforms and reproduces in test_decap traffic that uses 240.0.0.0/4 destinations.

How I did it

• Add a VPP patch that changes only the 240.0.0.0/4 default drop source from FIB_SOURCE_SPECIAL to FIB_SOURCE_DEFAULT_ROUTE.
• Leave multicast 224.0.0.0/4 as FIB_SOURCE_SPECIAL so multicast drop behavior is unchanged.
• Register the patch in vppbld/patches/series.

How I verified it

• git diff --check
• Verified the new patch applies cleanly to the pinned FD.io VPP source file from vppbld/vpp_version (src/vnet/fib/ip4_fib.c).

Fixes sonic-net/sonic-buildimage#27345

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).