smb: client: Clear sensitive stack data in smb2transport.c

Sensitive data like keys that are stored in stack-local arrays could
be leaked via the stack to the calling functions. There is no known
vulnerability for this right now, but it's good security style to
explicitly zeroize this sensitive material as soon as possible to
avoid that it could be exploited together with other bugs later.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>