Rate limit debugging stack traces

There have been issues in the past where excessive debug logging
to the console has resulted in significant performance impacts.
In the vast majority of these cases only a few stack traces are
required to diagnose the issue.  Therefore, stack traces dumped to
the console will now we limited to 5 every 60s.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Closes #374

module/spl/spl-kmem.c

@@ -725,7 +725,7 @@ kmem_alloc_debug(size_t size, int flags, const char *func, int line,
 		    "large kmem_alloc(%llu, 0x%x) at %s:%d (%lld/%llu)\n",
 		    (unsigned long long) size, flags, func, line,
 		    kmem_alloc_used_read(), kmem_alloc_max);
-		dump_stack();
+		spl_debug_dumpstack(NULL);
 	}
 
 	/* Use the correct allocator */