virtiofsd: Add support for FUSE_SYNCFS request
FUSE_SYNCFS allows the client to flush the host page cache.
v4: - based on upstream linux FUSE_SYNCFS
- added support for the '-o announce_submounts' case, i.e. client sends
a FUSE_SYNCFS request for each submount (identified by its root inode)
- adapted the case without '-o announce_submounts' so that syncfs() is
no longer called with lo->mutex held
v3: - track submounts and do per-submount syncfs() (Vivek)
- based on new version of FUSE_SYNCFS (still not upstream)
https://listman.redhat.com/archives/virtio-fs/2021-May/msg00025.html
v2: - based on new version of FUSE_SYNCFS
https://listman.redhat.com/archives/virtio-fs/2021-April/msg00166.html
- propagate syncfs() errors to client (Vivek)