drivers/dax: Expand lock scope to cover the use of addresses

The addition of PKS protection to dax read lock/unlock will require that
the address returned by dax_direct_access() be protected by this lock.

Correct the locking by ensuring that the use of kaddr and end_kaddr
are covered by the dax read lock/unlock.

Link: https://lore.kernel.org/r/20200717072056.73134-12-ira.weiny@intel.com
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>

drivers/dax/super.c

@@ -103,11 +103,11 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
 	id = dax_read_lock();
 	len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
 	len2 = dax_direct_access(dax_dev, pgoff_end, 1, &end_kaddr, &end_pfn);
-	dax_read_unlock(id);
 
 	if (len < 1 || len2 < 1) {
 		pr_info("%s: error: dax access failed (%ld)\n",
 				bdevname(bdev, buf), len < 1 ? len : len2);
+		dax_read_unlock(id);
 		return false;
 	}
 
@@ -137,6 +137,7 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
 		put_dev_pagemap(end_pgmap);
 
 	}
+	dax_read_unlock(id);
 
 	if (!dax_enabled) {
 		pr_info("%s: error: dax support not enabled\n",