fuse: extract fuse_fill_super_common()

fuse_fill_super() includes code to process the fd= option and link the
struct fuse_dev to the fd's struct file.  In virtio-fs there is no file
descriptor because /dev/fuse is not used.

This patch extracts fuse_fill_super_common() so that both classic fuse and
virtio-fs can share the code to initialize a mount.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>