Merge r196482 from head to stable/8:

Merge r196482 from head to stable/8:

  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

Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@196627 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f