md/raid10: avoid memory leak on error path during reshape.

If raid10 reshape fails to find somewhere to read a block
from, it returns without freeing memory...
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -4399,6 +4399,7 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr,
 		 * on all the target devices.
 		 */
 		// FIXME
+		mempool_free(r10_bio, conf->r10buf_pool);
 		set_bit(MD_RECOVERY_INTR, &mddev->recovery);
 		return sectors_done;
 	}