files-reg: Print file name on error

In case if there some error happened better to
know which exactly file failed on open, for example

 | Error (files-reg.c:535): Can't open file /lib64/libgcc_s-4.4.7-20120601.so.1 on restore: No such file or directory
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files-reg.c

@@ -532,7 +532,7 @@ static int do_open_reg(struct reg_file_info *rfi, void *arg)
 
 	fd = open(rfi->path, rfi->rfe->flags);
 	if (fd < 0) {
-		pr_perror("Can't open file on restore");
+		pr_perror("Can't open file %s on restore", rfi->path);
 		return fd;
 	}