pkg/mount.Mount: speedup (remove Mounted check)
pkg/mount.Mount: speedup (remove Mounted check)

This was added in PR #6669 (commit f87afda1234) because it was
otherwise impossible to do a re-mount of already mounted file system.

It is way better to just remove the Mounted() check altogether.

This change might potentially lead to multiple mounts to the same
mount point, so I audited all the users (except tests) and it looks
like no one is doing that:

 * volume/local maintains 'mounted' flag for every volume
 * pkg/chrootarchive already calls Mounted() before Mount()
   (so it actually parsed /proc/self/mountinfo twice, oops!)
 * daemon.mountVolumes() is called for docker cp only, and
   it is called once
 * daemon/graphdriver/zfs keeps track of 'mounted' status
 * daemon/graphdriver/devmapper: ditto
 * daemon.createSecretsDir() is only called once during container start

Surely I might have easily missed something so this needs a careful
review.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

moby/sys