Add syncfs() support
Fuse in kernel now has support to send FUSE_SYNCFS request. But there is no support in virtiofsd. Add the support. This is important because it is a data integrity operation.
Greg was doing syncfs() work (and was fixing issues related to submounts). I think submounts related issues he unearthed as part of syncfs work.
His last posting of syncfs patches is here.
https://lore.kernel.org/kvm/20210510191502.GA193692@horse/T/