[PATCH] Add linux-mm mailing list for memory management in MAINTAINERS file

Since I didn't know about the linux-mm mailing list until I spammed all
those that had their names anywhere in the mm directory, I'm sending this
patch to add the linux-mm mailing list to the MAINTAINERS file.

Also, since mm is so broad, it doesn't have a single person to maintain it,
and thus no maintainer is listed.  I also left the status as Maintained,
since it obviously is.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>