mount: dump a file system only if a mount point isn't overmounted

Here we try to enumerate all mount points and try to find one,
which allows us to dump content of a file system.

It's should be a root mount and its mount point should not be
overmounted.

We don't have a separate call-back to dump content of a file system.
fstype->dump() isn't always requires access to a mount point (e.g.
autofs), so we check overmounts in open_mountpoint().

$ cat /proc/61693/root/etc/redhat-release
Fedora release 23 (Twenty Three)

$ cat /proc/61692/mountinfo  | grep '\s/tmp'
234 199 0:57 / /tmp rw shared:97 master:76 - tmpfs tmpfs rw,size=131072k,nr_inodes=32768
235 234 0:57 /systemd-private-dd74de99e1104383aa7cd6e27d3d0b8a-httpd.service-uFqNHk/tmp /tmp rw,relatime shared:98 master:76 - tmpfs tmpfs rw,size=131072k,nr_inodes=32768

v2: return an error if we can't dump a file system
v3: try to find a mount point which allows to dump a file system
v4: check that children are not overmounted a target mount instead
of getting a mnt_id for a file descriptor.
v5: add a special error code for unreachable mount points

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>