gpib: fix double decrement of descriptor_busy in command_ioctl()

commit d1857f8296 ("gpib: fix use-after-free in IO ioctl handlers")
introduced a descriptor_busy reference counter to pin struct
gpib_descriptor across IO ioctl operations.  In command_ioctl(), the
error path inside the loop decrements descriptor_busy and breaks, but
execution then falls through to the unconditional decrement after the
loop, underflowing the counter to -1.

This re-enables the use-after-free that the original fix was meant to
prevent: a concurrent close_dev_ioctl() sees descriptor_busy == 0 on
an actively-used descriptor and frees it.

Remove the early decrement from the error path.  The post-loop
decrement already handles all exit paths, matching the correct pattern
used in read_ioctl() and write_ioctl().

Fixes: d1857f8296 ("gpib: fix use-after-free in IO ioctl handlers")
Reported-by: Ruikai Peng <ruikai@pwno.io>
Signed-off-by: Adam Crosser <adam.crosser@praetorian.com>
Link: https://patch.msgid.link/20260424123750.855863-1-adam.r.crosser@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpib/common/gpib_os.c

@@ -1010,7 +1010,6 @@ static int command_ioctl(struct gpib_file_private *file_priv,
 		userbuf += bytes_written;
 		if (retval < 0) {
 			atomic_set(&desc->io_in_progress, 0);
-			atomic_dec(&desc->descriptor_busy);
 
 			wake_up_interruptible(&board->wait);
 			break;