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

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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>