ALSA: pcm: Fix races among concurrent hw_params and hw_free calls

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

ALSA: pcm: Fix races among concurrent hw_params and hw_free calls

Currently we have neither proper check nor protection against the

concurrent calls of PCM hw_params and hw_free ioctls, which may result

in a UAF. Since the existing PCM stream lock can't be used for

protecting the whole ioctl operations, we need a new mutex to protect

those racy calls.

This patch introduced a new mutex, runtime->buffer_mutex, and applies

it to both hw_params and hw_free ioctl code paths. Along with it, the

both functions are slightly modified (the mmap_count check is moved

into the state-check block) for code simplicity.