fuse: allow sharing existing sb

Make it possible to create a new mount from a already working server.

Here's a detailed description of the problem from Jakob:

  "The background for this question is occasional problems we see with our
   fuse filesystem [1] and mount namespaces. On a usual client, we have
   system-wide, autofs managed mountpoints. When a new mount namespace is
   created (which can be done unprivileged in combination with user
   namespaces), it can happen that a mountpoint is used inside the new
   namespace but idle in the root mount namespace. So autofs unmounts the
   parent, system-wide mountpoint. But the fuse module stays active and
   still serves mountpoint in the child mount namespace. Because the fuse
   daemon also blocks other system wide resources corresponding to the
   mountpoint, this situation effectively prevents new mounts until the
   child mount namespaces closes.

   [1] https://github.com/cvmfs/cvmfs"

Reported-by: Jakob Blomer <jblomer@cern.ch>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>