gfs2: fix double destroy_workqueue error

In the Linux kernel, the following vulnerability has been resolved: gfs2: fix double destroy_workqueue error When gfs2_fill_super() fails, destroy_workqueue() is called within gfs2_gl_hash_clear(), and the subsequent code path calls destroy_workqueue() on the same work queue again. This issue can be fixed by setting the work queue pointer to NULL after the first destroy_workqueue() call and checking for a NULL pointer before attempting to destroy the work queue again.