interfaces: T3871: make the interface map survive a crash, not just a rename

Replacing the map was atomic but not durable: neither its contents nor the
rename were forced to the disk, so a machine losing power shortly after a
boot could come back to an empty or truncated file. Names are then derived
from the hardware again, which only gives the same answer where they happened
to follow slot order - anywhere else an address moves to another cable.

Force the contents out before the file is put in place, and the directory
entry after, so what comes back is either the names it had before or the ones
just written.

0 files changed, 0 insertions, 0 deletions