Fix zfs_arc_max minimum tuning

When setting `zfs_arc_max` its minimum value is allowed
to be 64 MiB.  There was an off-by-1 error which can matter
on tiny systems.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Zubrzycki <github@mid-earth.net>
Closes #7417

module/zfs/arc.c

@@ -6358,7 +6358,7 @@ arc_tuning_update(void)
 
 	/* Valid range: 64M - <all physical memory> */
 	if ((zfs_arc_max) && (zfs_arc_max != arc_c_max) &&
-	    (zfs_arc_max > 64 << 20) && (zfs_arc_max < allmem) &&
+	    (zfs_arc_max >= 64 << 20) && (zfs_arc_max < allmem) &&
 	    (zfs_arc_max > arc_c_min)) {
 		arc_c_max = zfs_arc_max;
 		arc_c = arc_c_max;