udp: Improve rmem accounting on loopback comms #1233

Job	Run time
build-minimal-sysroots (x86_64)	1m 7s
build-minimal-sysroots (riscv64)	1m 36s
build-minimal-sysroots (arm64)	1m 18s
build-toolchains (ubuntu-latest, x86_64, gnu)	28s
build-toolchains (ubuntu-latest, riscv64, gnu)	50s
build-toolchains (ubuntu-latest, arm64, gnu)	25s
build-toolchains (ubuntu-latest, llvm, all)	57s
build-llvm-riscv64	2m 42s
build	3m 4s
build-llvm	3m 3s
build-riscv	3m 29s
build-arm64	0s
build-llvm-arm64	0s
run-x86_64	0s
run-x86_64-llvm	0s
run-riscv	0s
run-riscv64-llvm	0s
	18m 59s

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