umount: umount -r segfault

umount: umount -r segfault

umount(8) segfaults when update incomplete mtab file after remount to
read-only (-r). For example autofs does not store info about
mountpoint to /etc/mtab file.

	# mount /dev/sda1 /mnt/test
	# sed -i -e 's:/dev/sda1 .*::g' /etc/mtab
	# cd /mnt/test
	# umount -r /mnt/test
	umount: /mnt/test busy - remounted read-only
	Segmentation fault

The command "umount -r" should not care about /etc/mtab if the related
mtab entry does not exist.

Reported-by: Paul Crawford <psc@sat.dundee.ac.uk>
Addresses: https://bugs.launchpad.net/bugs/579858
Signed-off-by: Karel Zak <kzak@redhat.com>