dm: update email address in MAINTAINERS

Update my email address to kernel.org to allow distinction between my
"upstream" and "Red" Hats.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: Mike Snitzer

MAINTAINERS

@@ -5587,7 +5587,7 @@ F:	include/linux/devm-helpers.h
 
 DEVICE-MAPPER  (LVM)
 M:	Alasdair Kergon <agk@redhat.com>
-M:	Mike Snitzer <snitzer@redhat.com>
+M:	Mike Snitzer <snitzer@kernel.org>
 M:	dm-devel@redhat.com
 L:	dm-devel@redhat.com
 S:	Maintained