ifnet: Restore curvnet earlier

This improves readability a little. As a side effect, a redundant
CURVNET_RESTORE is removed.

No functional change intended.

Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45595

(cherry picked from commit ef4f4a44d9134ddadede0e2e6e658d0688c5ab3c)