MAINTAINERS: change mhocko's email address to kernel.org

I am moving from mhocko@suse.cz to mhocko@kernel.org for kernel related
stuff.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>