net: sched: Allow changing default qdisc to FQ-PIE

Similar to fq_codel and the other qdiscs that can set as default,
fq_pie is also suitable for general use without explicit configuration,
which makes it a valid choice for this.

This is useful in situations where a painless out-of-the-box solution
for reducing bufferbloat is desired but fq_codel is not necessarily the
best choice. For example, fq_pie can be better for DASH streaming, but
there could be more cases where it's the better choice of the two simple
AQMs available in the kernel.

Signed-off-by: Danny Lin <danny@kdrag0n.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>