Prune metadata from ghost lists in arc_adjust_meta

To maintain a strict limit on the metadata contained in the arc, while
preventing the arc buffer headers from completely consuming the
"arc_meta_used" space, we need to evict metadata buffers from the arc's
ghost lists along with the regular lists.

This change modifies arc_adjust_meta such that it more closely models
the adjustments made in arc_adjust. "arc_meta_used" is used similarly to
"arc_size", and "arc_meta_limit" is used similarly to "arc_c".

Testing metadata intensive workloads (e.g. creating, copying, and
removing millions of small files and/or directories) has shown this
change to make a dramatic improvement to the hit rate maintained in the
arc. While I think there is still room for improvement, this is a big
step in the right direction.

In addition, zpl_free_cached_objects was made into a no-op as I'm not
yet sure how to properly implement that function.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2110

module/zfs/zpl_super.c

@@ -342,7 +342,7 @@ zpl_nr_cached_objects(struct super_block *sb)
 static void
 zpl_free_cached_objects(struct super_block *sb, int nr_to_scan)
 {
-	arc_adjust_meta(nr_to_scan * sizeof (znode_t), B_FALSE);
+	/* noop */
 }
 #endif /* HAVE_FREE_CACHED_OBJECTS */