setupLoop()'s Autoclear (LO_FLAGS_AUTOCLEAR) will destruct the
loopback device when all associated file descriptors are closed.
However this behavior didn't work before since setupLoop() was
returning a file name. The looppack device was destructed at
the end of the function when LoopParams had Autoclear = true.
Fixes#4969.
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>