NVMe: Enable BUILD_BUG_ON checks

Since _nvme_check_size() wasn't being called from anywhere, the compiler
was optimising it away ... along with all the link-time build failures
that would result if any of the structures were the wrong size.  Call it
from nvme_exit() for no particular reason.
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>

drivers/block/nvme-core.c

@@ -2912,6 +2912,7 @@ static void __exit nvme_exit(void)
 	unregister_blkdev(nvme_major, "nvme");
 	destroy_workqueue(nvme_workq);
 	BUG_ON(nvme_thread && !IS_ERR(nvme_thread));
+	_nvme_check_size();
 }
 
 MODULE_AUTHOR("Matthew Wilcox <willy@linux.intel.com>");