nvme-fabrics: prevent overriding of existing host

When first connecting a target using the "default" host parameters,
setting the hostid from the command line during a subsequent connection
establishment would override the "default" hostid parameter. This would
cause an existing connection that is already using the host definitions
to lose its hostid.

To address this issue, the code has been modified to allow only 1:1
mapping between hostnqn and hostid. This will maintain unambiguous host
identification. Any non 1:1 mapping will be rejected during connection
establishment.

Tested-by: Noam Gottlieb <ngottlieb@nvidia.com>
Reviewed-by: Israel Rukshin <israelr@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>