Enable zfs_arc_memory_throttle_disable by default

The zfs_arc_memory_throttle_disable module option was introduced
by commit 0c5493d to resolve a
memory miscalculation which could result in the txg_sync thread
spinning.

When this was first introduced the default behavior was left
unchanged until enough real world usage confirmed there were no
unexpected issues.  We've now reached that point.  Linux's
direct reclaim is working as expected so we're enabling this
behavior by default.

This helps pave the way to retire the spl_kmem_availrmem()
functionality in the SPL layer.  This was the only caller.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue openzfs#938

module/zfs/arc.c

@@ -190,7 +190,7 @@ unsigned long zfs_arc_meta_limit = 0;
 int zfs_arc_grow_retry = 0;
 int zfs_arc_shrink_shift = 0;
 int zfs_arc_p_min_shift = 0;
-int zfs_arc_memory_throttle_disable = 0;
+int zfs_arc_memory_throttle_disable = 1;
 int zfs_disable_dup_eviction = 0;
 int zfs_arc_meta_prune = 0;