Fix spurious mtab symlink error when /etc doesn't exist yet

symlink /proc/mounts /var/lib/docker/btrfs/subvolumes/1763d6602b8b871f0a79754f1cb0a31b3928bb95de5232b1b8c15c60fa1017f6-init/etc/mtab: no such file or directory

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #5134 from tianon/fix-mtab-symlink-error

Fix spurious mtab symlink error when /etc doesn't exist yet