xhci: sideband: Fix race condition in sideband unregister

xhci: sideband: Fix race condition in sideband unregister

Uttkarsh Aggarwal observed a kernel panic during sideband un-register
and found it was caused by a race condition between sideband unregister,
and creating sideband interrupters.
The issue occurrs when thread T1 runs uaudio_disconnect() and released
sb->xhci via sideband_unregister, while thread T2 simultaneously accessed
the now-NULL sb->xhci in xhci_sideband_create_interrupter() resulting in
a crash.

Ensure new endpoints or interrupter can't be added to a sidenband after
xhci_sideband_unregister() cleared the existing ones, and unlocked the
sideband mutex.
Reorganize code so that mutex is only taken and released once in
xhci_sideband_unregister(), and clear sb->vdev while mutex is taken.

Use mutex guards to reduce human unlock errors in code

Refuse to add endpoints or interrupter if sb->vdev is not set.
sb->vdev is set when sideband is created and registered.

Reported-by: Uttkarsh Aggarwal <uttkarsh.aggarwal@oss.qualcomm.com>
Closes: https://lore.kernel.org/linux-usb/20251028080043.27760-1-uttkarsh.aggarwal@oss.qualcomm.com
Fixes: de66754e9f80 ("xhci: sideband: add initial api to register a secondary interrupter entity")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://patch.msgid.link/20251107162819.1362579-4-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>