Flush enhancements and fixes

[minwooim]

This patchset introduces enhancements of fsync/fdatasync operations and fixes. The first patch added support for NVMe FLUSH commands in io_uring_cmd ioengine. The second one fixes issuing fsync/fdatasync even if the previous command is READ. As documentation says, fsync should sync the file after every N WRITE commands issued. The third one fixes not issuing fsync/fdatasync in trimwrite worklload. report.

Minwoo Im (4): io_uring: Add support FLUSH command io_u: ensure fsync only after write(s) io_u: Support fsync for --rw=trimwrite