checkpatch: check for incorrect permissions

Throw an error when a source file has been given execute permissions using
the mode change line present in git diffs.  Also alow the filename
matching to use the "diff" line in addition to the "+++" line, since the
mode change lines appear before any "+++" lines.

[apw@canonical.com: simplified filename logic slightly, added tests]
Cc: Andy Whitcroft <apw@canonical.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>