Make arc_prune() asynchronous

As described in the comment above arc_adapt_thread() it is critical
that the arc_adapt_thread() function never sleep while holding a hash
lock.  This behavior was possible in the Linux implementation because
the arc_prune() logic was implemented to be synchronous.  Under
illumos the analogous dnlc_reduce_cache() function is asynchronous.

To address this the arc_do_user_prune() function is has been reworked
in to two new functions as follows:

* arc_prune_async() is an asynchronous implementation which dispatches
the prune callback to be run by the system taskq.  This makes it
suitable to use in the context of the arc_adapt_thread().

* arc_prune() is a synchronous implementation which depends on the
arc_prune_async() implementation but blocks until the outstanding
callbacks complete.  This is used in arc_kmem_reap_now() where it
is safe, and expected, that memory will be freed.

This patch additionally adds the zfs_arc_meta_strategy module option
while allows the meta reclaim strategy to be configured.  It defaults
to a balanced strategy which has been proved to work well under Linux
but the illumos meta-only strategy can be enabled.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/arc.h

@@ -59,10 +59,16 @@ arc_done_func_t arc_getbuf_func;
 struct arc_prune {
 	arc_prune_func_t	*p_pfunc;
 	void			*p_private;
+	uint64_t		p_adjust;
 	list_node_t		p_node;
 	refcount_t		p_refcnt;
 };
 
+typedef enum arc_strategy {
+	ARC_STRATEGY_META_ONLY		= 0, /* Evict only meta data buffers */
+	ARC_STRATEGY_META_BALANCED	= 1, /* Evict data buffers if needed */
+} arc_strategy_t;
+
 typedef enum arc_flags
 {
 	/*