cifs: fix strcat buffer overflow and reduce raciness in smb21_set_oplock_level()

cifs: fix strcat buffer overflow and reduce raciness in smb21_set_oplock_level()

BugLink: https://bugs.launchpad.net/bugs/1824981
Change strcat to strncpy in the "None" case to fix a buffer overflow
when cinode->oplock is reset to 0 by another thread accessing the same
cinode. It is never valid to append "None" to any other message.

Consolidate multiple writes to cinode->oplock to reduce raciness.

Signed-off-by: Christoph Probst <kernel@probst.it>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>
(cherry picked from commit 6a54b2e002c9d00b398d35724c79f9fe0d9b38fb)
Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Acked-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>