CIFS: fix sha512 check in cifs_crypto_secmech_release

It seems this is a copy-paste error and that the proper variable to use
in this particular case is _sha512_ instead of _md5_.

Addresses-Coverity-ID: 1465358 ("Copy-paste error")
Fixes: 1c6614d229e7 ("CIFS: add sha512 secmech")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>