Improved vmem cached deadlock detection

Improved vmem cached deadlock detection

The entire goal of performing the slab allocations asynchronously
is to be able to detect when a vmalloc() deadlocks.  In this case,
and only this case, do we want to start allocating emergency objects.
The trick here is to minimize false positives because the overhead
of tracking emergency objects is far higher than normal slab objects.

With that goal in mind the code was reworked to be less sensitive
to slow allocations by increasing the wait time.  Once a cache is
is marked deadlocked all subsequent allocations which can not be
satisfied with existing cache objects will immediately allocate new
emergency objects.  This behavior persists until the asynchronous
allocation completes and clears the deadlocked flag.

The result of these tweaks is that far fewer emergency objects
get created which is important because this minimizes the cost of
releasing them latter in kmem_cache_free().

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>