Although commit 669de8bd ("kernel/workqueue: Use dynamic lockdep keys
for workqueues") unregisters dynamic lockdep keys when a workqueue is
destroyed, a side effect of that commit is that all stack traces
associated with the lockdep key are leaked when a workqueue is destroyed.
Fix this by storing each unique stack trace once. Other changes in this
patch are:

- Use NULL instead of { .nr_entries = 0 } to represent 'no trace'.
- Store a pointer to a stack trace in struct lock_class and struct
  lock_list instead of storing 'nr_entries' and 'offset'.

This patch avoids that the following program triggers the "BUG:
MAX_STACK_TRACE_ENTRIES too low!" complaint:

	#include <fcntl.h>
	#include <unistd.h>

	int main()
	{
		for (;;) {
			int fd = open("/dev/infiniband/rdma_cm", O_RDWR);
			close(fd);
		}
	}
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Yuyang Du <duyuyang@gmail.com>
Link: https://lkml.kernel.org/r/20190722182443.216015-4-bvanassche@acm.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>