* in the read vnode operator, check for IO_SYNC being set in the ioflag and
synchronously update the file's meta-data if appropriate.
* in the write vnode operator, update the appropriate checks for IO_SYNC being
set in the ioflag to reflect that IO_DSYNC is now inclusive-or'ed into
IO_SYNC, and require all IO_SYNC bits to be set for operations defined by
synchronized I/O file integrity completion but not by synchronized I/O data
integrity completion.
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