The io_uring_prep_fsync(3) function prepares an fsync request. The submission queue entry sqe is setup to
use the file descriptor fd that should get synced, with the modifier flags indicated by the flags
argument.
This function prepares an fsync request. It can act either like an fsync(2) operation, which is the
default behavior. If IORING_FSYNC_DATASYNC is set in the flags argument, then it behaves like
fdatasync(2). If no range is specified, the fd will be synced from 0 to end-of-file.
It's possible to specify a range to sync, if one is desired. If the off field of the SQE is set to non-
zero, then that indicates the offset to start syncing at. If len is set in the SQE, then that indicates
the size in bytes to sync from the offset. Note that these fields are not accepted by this helper, so
they have to be set manually in the SQE after calling this prep helper.