crimson/net: explicitly discard future from with_gate()

crimson/net: explicitly discard future from with_gate()

We are using with_gate() to wrap a slow operation when we want to start
it but don't want/need to wait it to be finished in the same function.
In this case, the future returned from with_gate() is discarded, and we
need to make sure there is another continuation responsible to waiting
for the gate closed.

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>