Usage of "fdatasync" breaks Mac build

[smklein]

Propolis (and the associated Omicron repo) primarily target illumos-based systems, but can compile and test for other platforms (such as Linux, and previously, macOS).

However, the addition of a call to fdatasync, which is only present for "linux-like" systems in the stdlib, breaks this compatibility:

propolis/propolis/src/block.rs

Lines 180 to 182 in dad75d2

	fn process_flush(&self) -> Result<BlockResult> {
	let res = unsafe { libc::fdatasync(self.fp.as_raw_fd()) };

This is visible in the Omicron github actions, where pulling Propolis forward makes this visible on Mac bots: https://github.com/oxidecomputer/omicron/pull/186/checks?check_run_id=3535422770#step:7:1089

Suggested steps forward:

• std::fs::File has methods for cross-platform fdatasync, which are presumably more portable.
• nix has a variant of this function which I believe is Mac-compatible.