if_clone: Make ifnet_detach_sxlock opaque to consumers

The change e133271fc1b5e introduced ifnet_detach_sxlock, and change
6d2a10d96fb5 widened its coverage, but there are still consumers,
net80211 and tuntap e.g., want it. Instead of sprinkling it everywhere,
make it opaque to consumers.

Out of tree drivers shall also benefit from this change.

Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D56298