sctp: mark sctp_do_peeloff static

sctp_do_peeloff is only used inside of net/sctp/socket.c,
so mark it static.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20250526054745.2329201-1-hch@lst.de


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