iflib: drain enqueued tasks before detaching from taskqgroup

The taskqgroup_detach function does not check if task is already enqueued when
detaching it. This may lead to kernel panic if enqueued task starts after
context state lock is destroyed. Ensure that the already enqueued admin tasks
are executed before detaching them.

The issue was discovered during validation of D16429. Unloading of if_ixlv
followed by immediate removal of VFs with iovctl -D may lead to panic on
NODEBUG kernel.

As well, check if iflib is in detach before enqueueing new admin or iov
tasks, to prevent new tasks from executing while the taskqgroup tasks
are being drained.

Submitted by:	Krzysztof Galazka <krzysztof.galazka@intel.com>
Reviewed by:	shurd@, erj@
Sponsored by:	Intel Corporation
Differential Revision:	https://reviews.freebsd.org/D17404

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