sctp: use the correct skb for security_sctp_assoc_request

Yi Chen reported an unexpected sctp connection abort, and it occurred when
COOKIE_ECHO is bundled with DATA Fragment by SCTP HW GSO. As the IP header
is included in chunk->head_skb instead of chunk->skb, it failed to check
IP header version in security_sctp_assoc_request().

According to Ondrej, SELinux only looks at IP header (address and IPsec
options) and XFRM state data, and these are all included in head_skb for
SCTP HW GSO packets. So fix it by using head_skb when calling
security_sctp_assoc_request() in processing COOKIE_ECHO.

v1->v2:
  - As Ondrej noticed, chunk->head_skb should also be used for
    security_sctp_assoc_established() in sctp_sf_do_5_1E_ca().

Fixes: e215dab1

 ("security: call security_sctp_assoc_request in sctp_sf_do_5_1D_ce")
Reported-by: Yi Chen <yiche@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://lore.kernel.org/r/71becb489e51284edf0c11fc15246f4ed4cef5b6.1649337862.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>