vnet: make VNET_FOREACH() always be a loop

[kp]

Summary

VNET_FOREACH() is a LIST_FOREACH if VIMAGE is set, but empty if it's
not. This means that users of the macro couldn't use 'continue' or
'break' as one would expect of a loop.

Change VNET_FOREACH() to be a loop in all cases (although one that is
fixed to one iteration if VIMAGE is not set).

Sponsored by: Rubicon Communications, LLC ("Netgate")

[glebius]

I wonder if compiler removes the loop and produces same code after the change.

[kp]

In D35739#810676, @glebius wrote:

I wonder if compiler removes the loop and produces same code after the change.

It does, yes, so this is a no-op, other than making the loop work the way a reader would expect it to, regardless of VIMAGE or ! VIMAGE.