fuse: simplify fuse_dev_ioctl_clone()

fuse: simplify fuse_dev_ioctl_clone()

Don't need to check if the new device file is already initialized, since
fuse_dev_install_with_pq() will do that anyway.

Make fuse_dev_install_with_pq() return a boolean value indicating success so
that fuse_dev_ioctl_clone() can return an error in case of failure.

Move aborting the connection (setting fc->connected to zero) to
fuse_dev_install(), because it is not needed when the clone ioctl fails.

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