pallet-message-queue: Fix max message size calculation #11902

Job	Run time
check-dependency-rules	9s
check-fail-ci	15s
check-markdown	15s
check-workspace	22s
preflight / preflight	41s
preflight / ci-versions	1m 11s
test-rust-features	2m 25s
check-rust-feature-propagation	2m 5s
check-toml-format	1m 2s
check-umbrella	2m 28s
fmt	1m 30s
All quick checks passed	1s
	12m 24s

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