aio: micro-optimize the lio_opcode assignments

[asomers]

Summary

This allows slightly more efficient opcode testing in-kernel. It is
transparent to userland, except to applications that sneakily submit
aio fsync or aio mlock operations via lio_listio, which has never been
documented, requires the use of deliberately undefined constants
(LIO_SYNC and LIO_MLOCK), and is arguably a bug.

Test Plan

existing AIO test suite

[tmunro]

Looks OK to me. I suppose we could also use bitmask style for LIO_SYNC and LIO_DATA_ONLY?

[jhb]

This seems ok to me, though it's not clear if the optimizations really buy anything in perf compared to the (minimal) obfuscation when reading.

I would very much like to see missing validation of opcodes to lio_listio added though, perhaps as a prior commit before this?

[asomers]

• aio: micro-optimize the lio_opcode assignments
• Rebase to include LIO_DSYNC

[asomers]

I rebased. I'll open a separate review to validate lio_listio opcodes.

[asomers]

• aio: micro-optimize the lio_opcode assignments
• Rebase to include LIO_DSYNC

[asomers]

Rebased to include lio_listio opcode validation.