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

Make sure to not leak hash data via the stack, clear it
with memzero_explicit() before leaving the function.

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