checkpatch: warn about using CONFIG_EXPERIMENTAL

This config item has not carried much meaning for a while now and is
almost always enabled by default.  As agreed during the Linux kernel
summit, it is being removed.  This will discourage future addition of
CONFIG_EXPERIMENTAL while it is being phased out.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>