MAINTAINERS: update rppt's email

Use my @kernel.org address

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mike Rapoport <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: rppt

MAINTAINERS

@@ -12400,7 +12400,7 @@ F:	include/uapi/linux/membarrier.h
 F:	kernel/sched/membarrier.c
 
 MEMBLOCK
-M:	Mike Rapoport <rppt@linux.ibm.com>
+M:	Mike Rapoport <rppt@kernel.org>
 L:	[email protected]
 S:	Maintained
 F:	Documentation/core-api/boot-time-mm.rst