sctp: Fix memory leak in sctp_sf_do_5_2_4_dupcook

In the implementation of sctp_sf_do_5_2_4_dupcook() the allocated
new_asoc is leaked if security_sctp_assoc_request() fails. Release it
via sctp_association_free().

Fixes: 2277c7cd ("sctp: Add LSM hooks")
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>