checkpatch: Handle long multi-line macros better.

Improve parsing of multiline macros which run beyond the available
diff context.  These beyond-the-horizon macros previously caused
two distinct naughty behaviors:
  - The scanner, confused by the trailing backslash, would grab
    the header of the next context hunk and treat it as the last
    line of the preceding macro.
  - The analyzer, unable to fully reduce the macro, would blame
    the patch for submitting an unbalanced or unprotected macro.

Change-Id: I6b7dd3d577c524d30b59dff7b20393bb5135f16d
Signed-off-by: Gregory Bean <gbean@codeaurora.org>
(cherry picked from commit ddd028c47b4d91aa9c0e97445eb584b2de367769)
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>