sctp: correct bounds check in sctp_setsockopt_auth_key

sctp: correct bounds check in sctp_setsockopt_auth_key

[ Upstream commit 328fc47ea0bcc27d9afa69c3ad6e52431cadd76c ]

The bonds check to prevent buffer overlflow was not exactly
right.  It still allowed overflow of up to 8 bytes which is
sizeof(struct sctp_authkey).

Since optlen is already checked against the size of that struct,
we are guaranteed not to cause interger overflow either.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>