block: add verifier for cmdline partition

I meet strange filesystem corruption issue recently, the reason
is there are overlaps partitions in cmdline partition argument.

This patch add verifier for cmdline partition, then if there are
overlaps partitions, cmdline_partition will log a warning. We don't
treat overlaps partition as a error:
"
Caizhiyong <caizhiyong@hisilicon.com> said:
Partition overlap was intentionally designed in this cmdline partition.
reference http://lists.infradead.org/pipermail/linux-mtd/2013-August/048092.html


"

Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>