sctp: move SCTP_PAD4 and SCTP_TRUNC4 to linux/sctp.h

sctp: move SCTP_PAD4 and SCTP_TRUNC4 to linux/sctp.h

Move these two macros from net/sctp/sctp.h to linux/sctp.h, so that
it will be enough to include only linux/sctp.h in nft_exthdr.c and
xt_sctp.c. It should not include "net/sctp/sctp.h" if a module does
not have a dependence on SCTP module.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Saeed Mahameed <saeed@kernel.org>
Link: https://lore.kernel.org/r/ef6468a687f36da06f575c2131cd4612f6b7be88.1668526821.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>