Allow multiple mounts of the same filesystem in some situations

[25pwn]

When trying to mount a filesystem that is already mounted, I get:

$ udisksctl mount -b "${device}"
Error mounting ${device}: GDBus.Error:org.freedesktop.UDisks2.Error.AlreadyMounted: Device ${device} is already mounted at `${path}'.

This disregards this fact from mount.2:

Since Linux 2.4 a single filesystem can be mounted at multiple mount points, and multiple mounts can be stacked on the same mount point.

udisks should allow multiple mounts of the same filesystem.