We now include btrfs in the snapshot driver test suite. This includes
the addition of parent links and name hashing into the btrfs driver.
We'll probably endup replacing this with a common metadata store, as
these relationships are generally identical between implementations.
A small bug was discovered in the delete implementation in the course
testing, so the btrfs package has been updated with a fix.
The overlay driver was modified accordingly with the btrfs driver to use
`Driver` as the exported type of each driver packge.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
Also remove the target from the Mount struct because it should not be
used at all. The target can be variable and set by a caller, not by the
snapshot drivers.
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
