DeviceMapper tasks in go use SetFinalizer to clean up C construct
counterparts in the C LVM library. While thats well and good, it relies
heavily on the exact interpretation of when the golang garbage collector
determines that an object is unreachable is subject to reclaimation.
While common sense would assert that for stack variables (which these DM
tasks always are), are unreachable when the stack frame in which they
are declared returns, thats not the case. According to this:

https://golang.org/pkg/runtime/#SetFinalizer

The garbage collector decides that, if a function calls into a
systemcall (which task.run() always will in LVM), and there are no
subsequent references to the task variable within that stack frame, then
it can be reclaimed. Those conditions are met in several devmapper.go
routines, and if the garbage collector runs in the middle of a
deviceMapper operation, then the task can be destroyed while the
operation is in progress, leading to crashes, failed operations and
other unpredictable behavior.

The fix is to use the KeepAlive interface:

https://golang.org/pkg/runtime/#KeepAlive

The KeepAlive method is effectively an empy reference that fools the
garbage collector into thinking that a variable is still reachable. By
adding a call to KeepAlive in the task.run() method, we can ensure that
the garbage collector won't reclaim a task object until its execution
within the deviceMapper C library is complete.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>