MAINTAINERS: clarify that only verified bugs should be submitted to security@

We're seeing a raise of automated reports from testing tools and reports
about address leaks that are not really exploitable as-is, many of which
do not represent an immediate risk justifying to work in closed places.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>