Enable Direct IO in truenas/zfs

Direct IO is enabled by default in the upstream zfs release but it conflicts with systemd-sysext (used to load NVIDIA drivers) in Scale Build, which is why we have disabled it in our truneas/zfs branches.

• The main issue preventing us from enabling Direct IO (where systemd-sysext fails to install extensions with Direct IO) is still open. However, a workaround is to use SYSTEMD_LOOP_DIRECT_IO=0.

• One of our earlier open issues causing a null pointer exception with systemd-sysext has been resolved.

• We also opened two more upstream issues. The exception related to the older kernel has been resolved. But the other issue where we observed differences in behavior with the recent kernel, remains open, which might affect us if applications using copy_file_range() that do not expect the errors.

• There was one additional issue related to Direct IO in Veeam, which was fixed within Veeam itself but it is dependent on statx patch, which needs to be pulled in our zfs branch.

This ticket tracks the effort to enable Direct IO before the GoldEye timeframe.