drbd: fix DRBD_VOLUME_MAX 65535 -> 65534

The protocol uses -1 as a reserved value for
'no specific volume', and since the protocol field
is a 16 bit unsigned value, -1 is converted to
65535. Therefore, limit the range of valid volume
numbers to [0, 65534].

Signed-off-by: Robert Altnoeder <[e-mail]>
Signed-off-by: Christoph Böhmwalder <[e-mail]>
Reviewed-by: Joel Colledge <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

include/linux/drbd_limits.h

···
 #define DRBD_MINOR_COUNT_DEF 32
 #define DRBD_MINOR_COUNT_SCALE '1'
 
-#define DRBD_VOLUME_MAX 65535
+#define DRBD_VOLUME_MAX 65534
 
 #define DRBD_DIALOG_REFRESH_MIN 0
 #define DRBD_DIALOG_REFRESH_MAX 600