FillBuf: Do not call poll_fill_buf twice #2418

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	45s
cargo minimal-versions build	31s
cargo build --target thumbv7m-none-eabi	31s
cargo +stable build	45s
cargo build --target thumbv6m-none-eabi	31s
cargo bench	51s
cargo +beta build	42s
cargo +nightly build	52s
cargo test (ubuntu-latest)	3m 3s
cargo +1.56 build	1m 10s
cargo test (macos-latest)	4m 52s
cargo clippy	27s
cargo test (windows-latest)	5m 6s
codegen	25s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 23s
cargo hack check --feature-powerset	2m 41s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 53s
cargo fmt	12s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 31s
cargo miri test	9m 1s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	4m 13s
cargo test -Z sanitizer=address	5m 35s
cargo test -Z sanitizer=memory	4m 2s
cargo test -Z sanitizer=thread	2m 50s
	1h 5m 52s

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