blk: add an upper sanity check on partition adding

65536 should be ludicrous anyway but without it we overflow the
memory computation doing the allocation and badness occurs.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Team-CodeCraft <kernelcraft@gmail.com>