nvme-multipath: Add visibility for round-robin io-policy

This patch helps add nvme native multipath visibility for round-robin
io-policy. It creates a "multipath" sysfs directory under head gendisk
device node directory and then from "multipath" directory it adds a link
to each namespace path device the head node refers.

For instance, if we have a shared namespace accessible from two different
controllers/paths then we create a soft link to each path device from head
disk node as shown below:

$ ls -l /sys/block/nvme1n1/multipath/
nvme1c1n1 -> ../../../../../pci052e:78/052e:78:00.0/nvme/nvme1/nvme1c1n1
nvme1c3n1 -> ../../../../../pci058e:78/058e:78:00.0/nvme/nvme3/nvme1c3n1

In the above example, nvme1n1 is head gendisk node created for a shared
namespace and the namespace is accessible from nvme1c1n1 and nvme1c3n1
paths.

For round-robin I/O policy, we could easily infer from the above output
that I/O workload targeted to nvme1n1 would toggle across paths nvme1c1n1
and nvme1c3n1.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>