block: merge partition-generic.c and check.c

Merge block/partition-generic.c and block/partitions/check.c into
a single block/partitions/core.c as the content is closely related
and both files are tiny.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>