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.

Reported-by:  <syzbot+d34c2a269ed512c531b0@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=d34c2a269ed512c531b0


Fixes: 30e388d5 ("gfs2: Switch to a per-filesystem glock workqueue")
Cc: stable@vger.kernel.org
Signed-off-by: Julian Sun <sunjunchao2870@gmail.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>