This is here only to support the Linux Kernel between versions
3.18 and 4.2. After that, this workaround is not needed anymore,
but it will work properly on both a kernel with and without the bug.

The bug is that when a process has a file open in an OverlayFS directory,
the information in /proc/<pid>/fd/<fd> and /proc/<pid>/fdinfo/<fd>
is wrong, so we grab that information from the mountinfo table instead.

This is done every time fill_fdlink is called.
We first check to see if the mnt_id and st_dev numbers currently match
some entry in the mountinfo table. If so, we already have the correct mnt_id
and no fixup is needed.

Then we proceed to see if there are any overlayFS mounted directories
in the mountinfo table. If so, we concatenate the mountpoint with the
name of the file, and stat the resulting path to check if we found the
correct device id and node number. If that is the case, we update the
mount id and link variables with the correct values.
Signed-off-by: Gabriel Guimaraes <gabriellimaguimaraes@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>