OpenZFS 9465 - ARC check for 'anon_size > arc_c/2' can stall the system

In the case of one pool being built on another pool, we want
to make sure we don't end up throttling the lower (backing)
pool when the upper pool is the majority contributor to dirty
data. To insure we make forward progress during throttling, we
also check the current pool's net dirty data and only throttle
if it exceeds zfs_arc_pool_dirty_percent of the anonymous dirty
data in the cache.

Authored by: Don Brady <don.brady@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

Porting Notes:
* The new global variables zfs_arc_dirty_limit_percent,
  zfs_arc_anon_limit_percent, and zfs_arc_pool_dirty_percent
  were intentially not added as tunable module parameters.

OpenZFS-issue: https://illumos.org/issues/9465
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d6a4c3ef
Closes #7749

include/sys/spa_impl.h

@@ -363,6 +363,11 @@ struct spa {
 	uint64_t	spa_errata;		/* errata issues detected */
 	spa_stats_t	spa_stats;		/* assorted spa statistics */
 	spa_keystore_t	spa_keystore;		/* loaded crypto keys */
+
+	/* arc_memory_throttle() parameters during low memory condition */
+	uint64_t	spa_lowmem_page_load;	/* memory load during txg */
+	uint64_t	spa_lowmem_last_txg;	/* txg window start */
+
 	hrtime_t	spa_ccw_fail_time;	/* Conf cache write fail time */
 	taskq_t		*spa_zvol_taskq;	/* Taskq for minor management */
 	taskq_t		*spa_prefetch_taskq;	/* Taskq for prefetch threads */