nvme: fix block device naming collision

The issue exists when multipath is enabled and the namespace is
shared, but all the other controller checks at nvme_is_unique_nsid()
are false. The reason for this issue is that nvme_is_unique_nsid()
returns false when is called from nvme_mpath_alloc_disk() due to an
uninitialized value of head->shared. The patch fixes it by setting
head->shared before nvme_mpath_alloc_disk() is called.

Fixes: 5974ea7c

 ("nvme: allow duplicate NSIDs for private namespaces")
Signed-off-by: Israel Rukshin <israelr@nvidia.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>