Call chdir("/") after chroot in pivot_root.

This change adds a chdir after chroot to avoid chroot breakouts.

BUG=chromium:517844
TEST=getcwd() returns "/" instead of "(unreachable)/.../root"

(Original patch by Luis Hector Chavez <lhchavez@google.com> at
https://chromium-review.googlesource.com/#/c/304362/)

Change-Id: If78f357636bcc1a3bfa71f377fef2ccc05b6dc0f
1 file changed