page-read: print errno in case of failure

(00.023059)   3898: Error (page-read.c:129): Can't read mapping page -1: Bad address
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

page-read.c

@@ -126,7 +126,7 @@ static int read_pagemap_page(struct page_read *pr, unsigned long vaddr, void *bu
 				vaddr, pr->cvaddr, lseek(pr->fd_pg, 0, SEEK_CUR));
 		ret = read(pr->fd_pg, buf, PAGE_SIZE);
 		if (ret != PAGE_SIZE) {
-			pr_err("Can't read mapping page %d\n", ret);
+			pr_perror("Can't read mapping page %d", ret);
 			return -1;
 		}
 	}