pk: Set LO_FLAGS_AUTOCLEAR on loop device
pk: Set LO_FLAGS_AUTOCLEAR on loop device

- Set autoclear flag when binding loop device, which asks the kernel
to unbind the loop device after the last fd to it is closed.  We still
try to unbind manually at shutdown, since the flag is only supported on
kernels >= 2.6.25.  On kernels that do support the flag, this will
only matter if we crash and therefore can't unbind by hand.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr