dm crypt: increase mempool reserve to better support swapping

Increase mempool size from 16 to 64 entries.  This increase improves
swap on dm-crypt performance.

When swapping to dm-crypt, all available memory is temporarily exhausted
and dm-crypt can only use the mempool reserve.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-crypt.c

@@ -181,7 +181,7 @@ struct crypt_config {
 	u8 key[0];
 };
 
-#define MIN_IOS        16
+#define MIN_IOS        64
 
 static void clone_init(struct dm_crypt_io *, struct bio *);
 static void kcryptd_queue_crypt(struct dm_crypt_io *io);