fix #27138800

commit c5d4355d10d414a96ca870b731756b89d068d57a upstream.

This patch adds functionality to perform flush from guest
to host over VIRTIO. We are registering a callback based
on 'nd_region' type. virtio_pmem driver requires this special
flush function. For rest of the region types we are registering
existing flush function. Report error returned by host fsync
failure to userspace.
Signed-off-by: NPankaj Gupta <pagupta@redhat.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Reviewed-by: NYang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: NXunlei Pang <xlpang@linux.alibaba.com>