fat: drop negative dentry on rename() path

Drop the negative dentry on rename() path, in order to make sure to
use the case sensitive name which is specified by user if this is for
creation.

For it, this uses newly added LOOKUP_RENAME_TARGET like LOOKUP_CREATE.
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

fs/fat/namei_vfat.c

@@ -78,7 +78,7 @@ static int vfat_revalidate_ci(struct dentry *dentry, struct nameidata *nd)
 	 * for creation.
 	 */
 	if (!(nd->flags & (LOOKUP_CONTINUE | LOOKUP_PARENT))) {
-		if (nd->flags & LOOKUP_CREATE)
+		if (nd->flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET))
 			return 0;
 	}