umount: do not hang with disconnected NFS mounts

umount: do not hang with disconnected NFS mounts

Since util-linux commit f4612577c942a3683b97632ad0b49671897c2070,
umount stat(2)s its argument to check whether it can be associated
with some loop device. This could hang with broken NFS mounts.

Fix by first checking mtab if umount's argument is mounted and only if
we fail, check if there is a loop device associated.

http://marc.info/?l=util-linux-ng&m=130924963804836&w=2

Reported-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Petr Uzel <petr.uzel@suse.cz>