pallet-message-queue: Fix max message size calculation #4139

Job	Run time
preflight / preflight	43s
preflight / ci-versions	1m 5s
test-full-crypto-feature	2m 34s
cargo-check-benches (master)	20m 10s
cargo-check-benches (current)	20m 28s
cargo-check-each-crate (1)	18m 47s
cargo-check-each-crate (2)	19m 14s
test-frame-ui	45m 43s
cargo-check-each-crate (3)	7m 3s
cargo-check-each-crate (4)	17m 18s
test-node-metrics	8m 23s
cargo-check-each-crate (5)	24m 9s
cargo-check-each-crate (6)	14m 31s
cargo-check-all-crate-macos	11m 15s
cargo-check-each-crate (7)	29m 56s
test-frame-examples-compile-to-wasm	3m 7s
test-deterministic-wasm	6m 11s
check-tracing	2m 10s
check-metadata-hash	6m 9s
node-bench-regression-guard	13s
All test misc tests passed	1s
	4h 19m 10s

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