smb3: fix setting SecurityFlags when encryption is required

Setting encryption as required in security flags was broken.
For example (to require all mounts to be encrypted by setting):

  "echo 0x400c5 > /proc/fs/cifs/SecurityFlags"

Would return "Invalid argument" and log "Unsupported security flags"
This patch fixes that (e.g. allowing overriding the default for
SecurityFlags  0x00c5, including 0x40000 to require seal, ie
SMB3.1.1 encryption) so now that works and forces encryption
on subsequent mounts.

Acked-by: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>