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>