Add conv=fdatasync flag to dd

[ryan_freqlabs.com]

Summary

The fdatasync flag performs an fdatasync(2) on the output file before closing it. This will be useful for the ZFS test suite.

Sponsored by: iXsystems, Inc.

[ryan_freqlabs.com]

I will check the result of close(2) in a follow-up patch as suggested in PR229616

[bcr]

OK from manpages, but bump the .Dd when committing for this content change.
Thank you!