fuse: don't require /dev/fuse fd to be kept open during mount

fuse: don't require /dev/fuse fd to be kept open during mount

With the new mount API the sequence of syscalls would be:

        fs_fd = fsopen("fuse", 0);
	snprintf(opt, sizeof(opt), "%i", devfd);
	fsconfig(fs_fd, FSCONFIG_SET_STRING, "fd", opt, 0);
	/* ... */
	fsconfig(fs_fd, FSCONFIG_CMD_CREATE, 0, 0, 0);

Current mount code just stores the value of devfd in the fs_context and
uses it in during FSCONFIG_CMD_CREATE, which is inelegant.

Instead grab a reference to the underlying fuse_dev, and use that during
the filesystem creation.

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