linux(4): Consolidate a FreeBSD interface names translation code

We have some amount of interface names translation functions which are
differs by bugs implementation. Consolidates it in a one place.

Fixup loopback interface names translation and use ifnet methods and
accessors, where possible.

Approved by: re (cperciva)
Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D38714
MFC after: 3 days
X-MFC with: 32fdc75fe7

(cherry picked from commit 3ab3c9c29cf0e5df8dbbaaf2003456445534bad8)
(cherry picked from commit a83551a52d1cfa8a756ef8dd298cab8042e27437)