lightnvm: pblk: use kvmalloc for metadata

There is no reason now not to use kvmalloc, so replace the internal
metadata allocation scheme.

Reviewed-by: Javier González <javier@javigon.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hans Holmberg <hans@owltronix.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>