nvme-multipath: add error handling support for add_disk()

We never checked for errors on add_disk() as this function
returned void. Now that this is fixed, use the shiny new
error handling.

Since we now can tell for sure when a disk was added, move
setting the bit NVME_NSHEAD_DISK_LIVE only when we did
add the disk successfully.

Nothing to do here as the cleanup is done elsewhere. We take
care and use test_and_set_bit() because it is protects against
two nvme paths simultaneously calling device_add_disk() on the
same namespace head.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>