Fix out-of-bound access in zfs_fillpage

The original code will do an out-of-bound access on pl[] during last
iteration.

 ==================================================================
 BUG: KASAN: stack-out-of-bounds in zfs_getpage+0x14c/0x2d0 [zfs]
 Read of size 8 by task tmpfile/7850
 page:ffffea00017c6dc0 count:0 mapcount:0 mapping:          (null) index:0x0
 flags: 0xffff8000000000()
 page dumped because: kasan: bad access detected
 CPU: 3 PID: 7850 Comm: tmpfile Tainted: G           OE   4.6.0+ #3
  ffff88005f1b7678 0000000006dbe035 ffff88005f1b7508 ffffffff81635618
  ffff88005f1b7678 ffff88005f1b75a0 ffff88005f1b7590 ffffffff81313ee8
  ffffea0001ae8dd0 ffff88005f1b7670 0000000000000246 0000000041b58ab3
 Call Trace:
  [<ffffffff81635618>] dump_stack+0x63/0x8b
  [<ffffffff81313ee8>] kasan_report_error+0x528/0x560
  [<ffffffff81278f20>] ? filemap_map_pages+0x5f0/0x5f0
  [<ffffffff813144b8>] kasan_report+0x58/0x60
  [<ffffffffc12250dc>] ? zfs_getpage+0x14c/0x2d0 [zfs]
  [<ffffffff81312e4e>] __asan_load8+0x5e/0x70
  [<ffffffffc12250dc>] zfs_getpage+0x14c/0x2d0 [zfs]
  [<ffffffffc1252131>] zpl_readpage+0xd1/0x180 [zfs]

  [<ffffffff81353c3a>] SyS_execve+0x3a/0x50
  [<ffffffff810058ef>] do_syscall_64+0xef/0x180
  [<ffffffff81d0ee25>] entry_SYSCALL64_slow_path+0x25/0x25
 Memory state around the buggy address:
  ffff88005f1b7500: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  ffff88005f1b7580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 >ffff88005f1b7600: 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 f4
                                                                 ^
  ffff88005f1b7680: f4 f4 f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00
  ffff88005f1b7700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ==================================================================

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#4705
Issue openzfs#4708

module/zfs/zfs_vnops.c

@@ -4271,10 +4271,10 @@ zfs_fillpage(struct inode *ip, struct page *pl[], int nr_pages)
 	 * Iterate over list of pages and read each page individually.
 	 */
 	page_idx = 0;
-	cur_pp   = pl[0];
 	for (total = io_off + io_len; io_off < total; io_off += PAGESIZE) {
 		caddr_t va;
 
+		cur_pp = pl[page_idx++];
 		va = kmap(cur_pp);
 		err = dmu_read(os, zp->z_id, io_off, PAGESIZE, va,
 		    DMU_READ_PREFETCH);
@@ -4285,7 +4285,6 @@ zfs_fillpage(struct inode *ip, struct page *pl[], int nr_pages)
 				err = SET_ERROR(EIO);
 			return (err);
 		}
-		cur_pp = pl[++page_idx];
 	}
 
 	return (0);