lib/igt_core: reinitialize log_buffer_mutex in child process

For multi-process scenario intel-allocator is dedicated thread which
handles clients (children processes) allocations. Main thread which
spawns children may hit point of time where allocator thread locked
some mutex. One of it is log_buffer_mutex used in igt_log(). Children
which would try to call igt_info/warn/debug() family will stuck as
this mutex is already locked.

Reinitialize this mutex in igt_fork() to avoid deadlocks in allocator
scenarios.

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Dominik Karol Piątkowski <dominik.karol.piatkowski@intel.com>
Cc: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Reviewed-by: Dominik Karol Piątkowski <dominik.karol.piatkowski@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Link: https://lore.kernel.org/r/20250317113146.560156-1-zbigniew.kempczynski@intel.com


Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>