Obey arc_meta_limit default size when changing arc_max

When decreasing the maximum ARC size preserve the 3/4 default
ratio for the arc_meta_limit.  Otherwise, the arc_meta_limit
may be set the same as arc_max.

Signed-off-by: AndCycle <andcycle@andcycle.idv.tw>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#4001

module/zfs/arc.c

@@ -5245,7 +5245,7 @@ arc_tuning_update(void)
 		arc_c_max = zfs_arc_max;
 		arc_c = arc_c_max;
 		arc_p = (arc_c >> 1);
-		arc_meta_limit = MIN(arc_meta_limit, arc_c_max);
+		arc_meta_limit = MIN(arc_meta_limit, (3 * arc_c_max) / 4);
 	}
 
 	/* Valid range: 32M - <arc_c_max> */