Proposed fix for low memory ZFS deadlocks

Deadlocks in the zvol were observed when one of the ZFS threads
performing IO trys to allocate memory while the system is low
on memory.  The low memory condition causes dirty pages to be
synced to the zvol but this can't progress because the original
thread is blocked waiting on a memory allocation.  Thus we end
up deadlocking.

A proper solution proposed by Wizeman is to change KM_SLEEP from
GFP_KERNEL top GFP_NOFS.  This will prevent the memory allocation
which is trying to allocate memory from forcing a sync to the
zvol in shrink_page_list()->pageout().

The down side to all of this is that we are using a pretty big
hammer by changing KM_SLEEP.  This change means ALL of the zfs
memory allocations will be until to trigger dirty data to be
synced.  The caller still should be able to reclaim memory from
the various slab caches.  We will be totally dependent of other
kernel processes which happen to be running and a small number
of asynchronous reclaim threads to trigger the reclaim of dirty
data pages.  This should be OK but I think we may see some
slightly longer allocation times when under memory pressure.

We shall see.

include/sys/kmem.h

@@ -42,7 +42,7 @@
 /*
  * Memory allocation interfaces
  */
-#define KM_SLEEP                        GFP_KERNEL
+#define KM_SLEEP                        GFP_NOFS
 #define KM_NOSLEEP                      GFP_ATOMIC
 #undef  KM_PANIC                        /* No linux analog */
 #define KM_PUSHPAGE                     (KM_SLEEP | __GFP_HIGH)