sctp: switch sctp_setsockopt_auth_key to use memzero_explicit

Switch from kzfree to sctp_setsockopt_auth_key + kfree to prepare for
moving the kfree to common code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>