intel_th: msu: Prevent freeing buffers while locked windows exist

intel_th: msu: Prevent freeing buffers while locked windows exist

We already prevent freeing buffers via sysfs interface in case there are
existing users or if trace is active. Treat the existence of locked windows
similarly and return -EBUSY on attempts to free the buffer. When the last
window is unlocked, the freeing will succeed.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190705141425.19894-5-alexander.shishkin@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>