CIFS: Fix a possible memory corruption in push locks

[ Upstream commit e3d240e9d505fc67f8f8735836df97a794bbd946 ]

If maxBuf is not 0 but less than a size of SMB2 lock structure
we can end up with a memory corruption.

Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>