CVE-2026-72482: gpib: fix double decrement of descriptor_busy in command_ioctl()

In the Linux kernel, the following vulnerability has been resolved:

gpib: fix double decrement of descriptorbusy in commandioctl()

commit d1857f8296dc ("gpib: fix use-after-free in IO ioctl handlers") introduced a descriptorbusy reference counter to pin struct gpibdescriptor across IO ioctl operations. In commandioctl(), the error path inside the loop decrements descriptorbusy 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 closedevioctl() sees descriptorbusy == 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 readioctl() and writeioctl().