mount: fix "mount -a" for auto-clear loopdevs and /proc/mounts

mount: fix "mount -a" for auto-clear loopdevs and /proc/mounts

The "mount -a" uses /etc/mtab to detect already mounted file systems
from /etc/fstab -- this check requires the same FS name (1st field)
in fstab and mtab.

On systems with enabled auto-clear loop devies or systems without
regular mtab  (symlink to /proc/mounts) there is /dev/loopN rather
than image filename in mtab. For example:

  fstab:
     /mnt/store/foo.ISO /mnt/image auto	defaults 0 0

  mtab:
     /dev/loop0 /mnt/image iso9960 rw 0 0

We have to scan all available loop devices to check if some of the
devices is not associated with the image file from fstab.

Addresses: http://bugzilla.redhat.com/show_bug.cgi?id=618957
Signed-off-by: Karel Zak <kzak@redhat.com>