MAINTAINERS: add Karsten as SMC maintainer

Add Karsten as additional maintainer for Shared Memory Communications
(SMC) Sockets.

Acked-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>