smb3: display volume serial number for shares in /proc/fs/cifs/DebugData

It can be helpful for debugging.  According to MS-FSCC:

"A 32-bit unsigned integer that contains the serial number of the
volume. The serial number is an opaque value generated by the file
system at format time"

Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Pavel Shilovsky <pshilov@microsoft.com>