cifs: have cifs_cleanup_volume_info not take a double pointer

...as that makes for a cumbersome interface. Make it take a regular
smb_vol pointer and rely on the caller to zero it out if needed.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastryyy@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>