sctp: change to include linux/sctp.h in net/sctp/checksum.h

Currently "net/sctp/checksum.h" including "net/sctp/sctp.h" is
included in quite some places in netfilter and openswitch and
net/sched. It's not necessary to include "net/sctp/sctp.h" if
a module does not have dependence on SCTP, "linux/sctp.h" is
the right one to include.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Saeed Mahameed <saeed@kernel.org>
Link: https://lore.kernel.org/r/ca7ea96d62a26732f0491153c3979dc1c0d8d34a.1668526793.git.lucien.xin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sctp/checksum.h

@@ -24,7 +24,7 @@
 #define __sctp_checksum_h__
 
 #include <linux/types.h>
-#include <net/sctp/sctp.h>
+#include <linux/sctp.h>
 #include <linux/crc32c.h>
 #include <linux/crc32.h>