CVE-2026-98742: ALSA: seq: Serialize compat port-info ioctls

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

ALSA: seq: Serialize compat port-info ioctls

The native sequencer ioctl path serializes handler calls with client->ioctlmutex, but the translated port-info compat path invokes the same handlers through sndseqkernelclientctl() without taking that mutex.

This lets concurrent compat CREATEPORT requests pass the port-count check before any request reaches the serialized insertion. The computed integer port index can then exceed the address field range and wrap to an existing index. Subsequent subscriber teardown can resolve the duplicate address to the wrong port and access a freed subscriber.

Take ioctlmutex while dispatching converted port-info requests, matching the native ioctl path. All translated port-info commands share this helper, so their accesses to the client port state are serialized as well.