i40e: Fix off by one in i40e_dbg_command_write

We assume that the resulting buffer is zero terminated when we then
re-use it. The sscanf is limited to 512 bytes but needs to be 511
to allow for a terminator.

One of a set of problems noted by Jackie Chang

Signed-off-by: Alan Cox <alan@linux.intel.com>
Acked-by: Shannon Nelson <Shannon.nelson@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_debugfs.c

@@ -1547,7 +1547,7 @@ static ssize_t i40e_dbg_command_write(struct file *filp,
 		if (strncmp(cmd_buf, "add", 3) == 0)
 			add = true;
 		cnt = sscanf(&cmd_buf[13],
-			     "%hx %2hhx %2hhx %hx %2hhx %2hhx %hx %x %hd %512s",
+			     "%hx %2hhx %2hhx %hx %2hhx %2hhx %hx %x %hd %511s",
 			     &fd_data.q_index,
 			     &fd_data.flex_off, &fd_data.pctype,
 			     &fd_data.dest_vsi, &fd_data.dest_ctl,