Rather than using IFNET_RLOCK() when iterating over (and modifying) the
ifnet list during if_ef load, directly acquire the ifnet_sxlock
exclusively. That way when if_alloc() recurses the lock, it's a write
recursion rather than a read->write recursion.

This code structure is arguably a bug, so add a comment indicating that
this is the case. Post-8.0, we should fix this, but this commit
resolves panic-on-load for if_ef.

Discussed with: bz, julian
Reported by: phk
MFC after: 3 days