Illumos 6033 - arc_adjust() should search MFU lists

6033 arc_adjust() should search MFU lists for oldest
buffer when adjusting MFU size
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: Xin Li <delphij@delphij.net>
Reviewed by: Prakash Surya <me@prakashsurya.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>

References:
  https://www.illumos.org/issues/6033
  illumos/illumos-gate@31c46cf

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3545

module/zfs/arc.c

@@ -2880,7 +2880,7 @@ arc_adjust(void)
 	 */
 	target = arc_size - arc_c;
 
-	if (arc_adjust_type(arc_mru) == ARC_BUFC_METADATA &&
+	if (arc_adjust_type(arc_mfu) == ARC_BUFC_METADATA &&
 	    arc_meta_used > arc_meta_min) {
 		bytes = arc_adjust_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
 		total_evicted += bytes;