MAINTAINERS: add Jan as SMC maintainer

Add Jan as maintainer for Shared Memory Communications (SMC)
Sockets.
Acked-by: Jan Karcher <jaka@linux.ibm.com>
Acked-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Wenjia Zhang <wenjia@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

MAINTAINERS

@@ -18487,6 +18487,7 @@ F:	drivers/misc/sgi-xp/
 SHARED MEMORY COMMUNICATIONS (SMC) SOCKETS
 M:	Karsten Graul <kgraul@linux.ibm.com>
 M:	Wenjia Zhang <wenjia@linux.ibm.com>
+M:	Jan Karcher <jaka@linux.ibm.com>
 L:	linux-s390@vger.kernel.org
 S:	Supported
 W:	http://www.ibm.com/developerworks/linux/linux390/