gpib: fix use-after-free in IO ioctl handlers

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

gpib: fix use-after-free in IO ioctl handlers

The IBRD, IBWRT, IBCMD, and IBWAIT ioctl handlers use a gpib_descriptor
pointer after board->big_gpib_mutex has been released. A concurrent
IBCLOSEDEV ioctl can free the descriptor via close_dev_ioctl() during
this window, causing a use-after-free.

The IO handlers (read_ioctl, write_ioctl, command_ioctl) explicitly
release big_gpib_mutex before calling their handler. wait_ioctl() is
called with big_gpib_mutex held, but ibwait() releases it internally
when wait_mask is non-zero. In all four cases, the descriptor pointer
obtained from handle_to_descriptor() becomes unprotected.

Fix this by introducing a kernel-only descriptor_busy reference count
in struct gpib_descriptor. Each handler atomically increments
descriptor_busy under file_priv->descriptors_mutex before releasing the
lock, and decrements it when done. close_dev_ioctl() checks
descriptor_busy under the same lock and rejects the close with -EBUSY
if the count is non-zero.

A reference count rather than a simple flag is necessary because
multiple handlers can operate on the same descriptor concurrently
(e.g. IBRD and IBWAIT on the same handle from different threads).

A separate counter is needed because io_in_progress can be cleared from
unprivileged userspace via the IBWAIT ioctl (through general_ibstatus()
with set_mask containing CMPL), which would allow an attacker to bypass
a check based solely on io_in_progress. The new descriptor_busy
counter is only modified by the kernel IO paths.

The lock ordering is consistent (big_gpib_mutex -> descriptors_mutex)
and the handlers only hold descriptors_mutex briefly during the lookup,
so there is no deadlock risk and no impact on IO throughput.