sockets: make pr_aio_queue the default method

Call it directly instead of each time calling pr_aio_queue_notsupp() and
then doing the actual job. The only user of non default method was
cxgbe(4). It calls now into the default method in case of own method
failure. This preserves existing behavior.

Reviewed by: markj, jhb
Differential Revision: https://reviews.freebsd.org/D48863