fusefs: don't fake the mountpoint's stat info before FUSE_INIT completes

Ever since the first GSoC contribution, fusefs has had a curious
behavior. If the daemon hasn't finished responding to FUSE_INIT,
fuse_vnop_getattr would reply to VOP_GETATTR requests for the mountpoint
by returning all zeros. I don't know why. It isn't necessary for
unmounting, even if the daemon is dead.

Delete that behavior. Now VOP_GETATTR for the mountpoint will wait for
the daemon to be ready, just like it will for any other vnode.

Reported by: Vassili Tchersky
Sponsored by: ConnectWise
Differential Revision: https://reviews.freebsd.org/D50800

(cherry picked from commit 9e682c426093a3afc0f609a1f04048b0beb021ec)