staging: vcsm-cma: Fixup the alloc code handling of kernel_id

The allocation code had been copied in from an old branch prior
to having added the IDR for 64bit support. It was therefore pushing
a pointer into the kernel_id field instead of an IDR handle, the
lookup therefore failed, and we never released the buffer.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>