libext2fs/ismounted.c: check device id in advance to skip false device names
If there is a trickster which tries to use device names as the mount
device for pseudo-file systems, the resulting /proc/mounts can confuse
ext2fs_check_mount_point(). (So far as I can tell, there's no good
reason to do this, but sysadmins do the darnest things.)
An example of this might be the following /proc/mounts excerpt:
This is created via "mount -t tmpfs /dev/sdb /mnt2" followed via
"mount -t ext4 /dev/sdb /mnt". (Normally, a sane mount of tmpfs would
use something like "mount -t tmpfs tmpfs /mnt2".)
Fix this by double checking the st_rdev of the claimed mountpoint and
match it with the dev_t of the device. (Note that the GNU HURD
doesn't support st_rdev, so we can't solve this problem for the HURD.)