pagemap: Use pread to read pagemap entries

When reading pagemaps, we read it from specific position. To
do it, we called lseek, then read. Fortunetely, there's a
syscall that does both things in one call -- pread. Since
we don't need to keep pagemap's position for further reads,
it perfectly suits our needs.

This removes 75% of lseek calls when dumping basic container.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mem.c

@@ -105,17 +105,12 @@ static int generate_iovs(struct vma_area *vma, int pagemap, struct page_pipe *pp
 {
 	unsigned long pfn, nr_to_scan;
 	unsigned long pages[2] = {};
-	u64 aux;
-
-	aux = vma->vma.start / PAGE_SIZE * sizeof(*map);
-	if (lseek(pagemap, aux, SEEK_SET) != aux) {
-		pr_perror("Can't rewind pagemap file");
-		return -1;
-	}
+	u64 from, len;
 
 	nr_to_scan = vma_area_len(vma) / PAGE_SIZE;
-	aux = nr_to_scan * sizeof(*map);
-	if (read(pagemap, map, aux) != aux) {
+	from = vma->vma.start / PAGE_SIZE * sizeof(*map);
+	len = nr_to_scan * sizeof(*map);
+	if (pread(pagemap, map, len, from) != len) {
 		pr_perror("Can't read pagemap file");
 		return -1;
 	}