Uses actual number of items in generate_index_for_slot() (#2919) #353

Job	Run time
benchmark (solana-sdk, cargo +$rust_nightly bench -p solana-sdk)	1m 27s
benchmark (solana-runtime, cargo +$rust_nightly bench -p solana-runtime)	1m 13s
benchmark (solana-gossip, cargo +$rust_nightly bench -p solana-gossip)	2m 58s
benchmark (solana-poh, cargo +$rust_nightly bench -p solana-poh)	1m 30s
benchmark (solana-core, cargo +$rust_nightly bench -p solana-core)	5m 34s
benchmark (sbf, make -C programs/sbf all, cargo +$rust_nightly bench --manifest-path programs/sbf...	1m 29s
benchmark (solana-accounts-db, cargo +$rust_nightly bench -p solana-accounts-db --bench accounts_...	13m 28s
	27m 39s

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