libmount: don't setup new loopdev on remount with regular mtab

libmount: don't setup new loopdev on remount with regular mtab

[chroot-i486] root:/tmp$ mount -oloop /tmp/blob /tmp/mnt
[chroot-i486] root:/tmp$ mount -o remount,loop,ro /tmp/mnt
[chroot-i486] root:/tmp$ mount
/dev/loop0 on /tmp/mnt type ext2 (ro,loop)
[chroot-i486] root:/tmp$ losetup -a
/dev/loop0: [2051]:306957 (/tmp/blob)
/dev/loop1: [0005]:2218 (/dev/loop0)
[chroot-i486] root:/tmp$ umount /tmp/mnt
umount: /tmp/mnt: filesystem umounted, but mount(8) failed: Device or
resource busy
[chroot-i486] root:/tmp$ losetup -d /dev/loop1
[chroot-i486] root:/tmp$ losetup -d /dev/loop0

Reported-by: g.esp@free.fr
Signed-off-by: Karel Zak <kzak@redhat.com>