symlink /etc/mtab and /proc/mounts

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
devmapper: Pass info rather than hash to activateDeviceIfNeeded

There is no need to look this up again, we have it already in all callers.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Pass info rather than hash to deleteDevice

All the callers already have the info, no need for an extra lookup.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Pass info rather than hash to deactivateDevice()

We already have the info in most cases, no need to look this up multiple times.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Pass info rather than hash to setInitialized

We already have this at the caller, no need to look up again.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Add lookupDevice() helper

This centralizes the lookup of devices so it is only done in one place.
This will be needed later when we change the locking for it.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Add lock to protext Devices map

Currently access to the Devices map is serialized by the main
DeviceSet lock, but we need to access it outside that lock, so we
add a separate lock for this and grab that everywhere we modify
or read the map.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Avoid AB-BA deadlock

We currently drop the global lock while holding a per-device lock when
waiting for device removal, and then we re-aquire it when the sleep is done.
This is causing a AB-BA deadlock if anyone at the same time tries to do any
operation on that device like this:

thread A:             thread B
grabs global lock
grabs device lock
releases global lock
sleeps
                      grabs global lock
                      blocks on device lock
wakes up
blocks on global lock

To trigger this you can for instance do:

ID=`docker run -d fedora sleep 5`
cd /var/lib/docker/devicemapper/mnt/$ID
docker wait $ID
docker rm $ID &
docker rm $ID

The unmount will fail due to the mount being busy thus causing the
timeout and the second rm will then trigger the deadlock.

We fix this by adding a lock ordering such that the device locks
are always grabbed before the global lock. This is safe since the
device lookups now have a separate lock.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4779 from vieux/symlink_mtab

symlink /etc/mtab and /proc/mounts
Merge pull request #4951 from alexlarsson/dm-fix-deadlock

Fix AB-BA deadlock in devicemapper backend