macros: add KPATCH_REVERT_FUNCTION macro

[jpoimboe]

Add a KPATCH_REVERT_FUNCTION macro for forcing a function to be patched
with its original contents. This is needed for the case where the patch
is a cumulative upgrade to a previous patch which modified the function,
and now the current patch needs to revert that change.

This is basically a copy/paste/modify job from KPATCH_IGNORE_FUNCTION
and kpatch_mark_ignored_functions_same().

[jpoimboe]

This probably needs a patch author guide update as well, to document the steps:

1. compile v1 and note the list of changed functions

2. compile v2 (without KPATCH_REVERT_FUNCTION macro) and note the
   list of changed functions

3. for those functions which changed in v1 and did not change in v2,
   annotate them with KPATCH_REVERT_FUNCTION

[jpoimboe]

We had problems testing this macro in a case where the function symbol had an isra suffix. Adding a reference to the function name caused gcc to change its inlining decision for the function, causing the isra suffix to get removed, and then a seg fault happened.

So I'm going to drop this patch for now. We can use #716 to track the fact that we'll need to bring back the replace feature, or something comparable, to make cumulative update reverts more straightforward.