dm era: Use correct value size in equality function of writeset tree

Fix the writeset tree equality test function to use the right value size
when comparing two btree values.

Fixes: eec40579 ("dm: add era target")
Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Reviewed-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-era-target.c

@@ -389,7 +389,7 @@ static void ws_dec(void *context, const void *value)
 
 static int ws_eq(void *context, const void *value1, const void *value2)
 {
-	return !memcmp(value1, value2, sizeof(struct writeset_metadata));
+	return !memcmp(value1, value2, sizeof(struct writeset_disk));
 }
 
 /*----------------------------------------------------------------*/