krpc: Get rid of KRPC_VNET macros

When the krpc was vnet'd, the VNET macros were hidden
behind macros that had the KRPC_ prefix on them.
This was done because, at the time, it was thought
that something other than vnet might be used for this.
That has not happened and probably will not happen,
so this patch replaces these obscuring macros with
the regular vnet ones.

There should be no semantics change caused by
this commit.

(cherry picked from commit d8d9e1cf5e055433565a7e686bc096a8494d2bb8)