overlay/018: test lower hardlinks re-unite on copy up

Test that when two lower hardlinks are copied up, they end up
as two upper hardlinks of the same upper inode.

Drop caches before copy up so there is no knowledge of the
copied up hardlink in inode/dcache.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/overlay/018

@@ -111,5 +111,15 @@ echo "== After mount cycle =="
 cat $FILES
 check_ino_nlink $tmp.after_one $tmp.after_cycle
 
+# Drop caches to get the copied up hardlink out of cache
+echo 3 > /proc/sys/vm/drop_caches
+
+# Modify content of the other hardlink
+echo "two" >> $foo
+
+echo "== After write two =="
+cat $FILES
+check_ino_nlink $tmp.after_one $tmp.after_two
+
 status=0
 exit

tests/overlay/018.out

@@ -12,3 +12,10 @@ zero
 one
 zero
 one
+== After write two ==
+zero
+one
+two
+zero
+one
+two