iflib: add support for admin completion queues

For interfaces with admin completion queues, introduce a new devmethod
IFDI_ADMIN_COMPLETION_HANDLE and a corresponding flag IFLIB_HAS_ADMINCQ.

This provides an option for handling any admin cq logic, which cannot be
run from an interrupt context.

Said method is called from within iflib's admin task, making it safe to
sleep.

Reviewed by: mmacy
Submitted by: Artur Rojek <ar@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D28708