[SPDK] RFC: NVMf namespace masking

Hi all,

I would be happy to get some feedback on my NVMf target namespace masking 
implementation using attach/detach:
https://review.spdk.io/gerrit/c/spdk/spdk/+/7821

The patch introduces namespace masking for NVMe-over-fabrics
targets by allowing to (dynamically) attach and detach
controllers to/from namespaces, cf. NVMe spec 1.4 - section 6.1.4.
Since SPDK only supports the dynamic controller model a new
controller is allocated on every fabric connect command.
This allows to attach/detach controllers of a specific
host NQN to/from a namespace. A host can only perform
operations to an active namespace. Inactive namespaces can
be listed (not supported by SPDK) but no additional
information can be retrieved:
"Unless otherwise noted, specifying an inactive NSID in a
command that uses the Namespace Identifier (NSID) field shall
cause the controller to abort the command with status
Invalid Field in Command" - NVMe spec 1.4 - section 6.1.5
Note that this patch does not implement the NVMe namespace
attachment command but allows to attach/detach via RPCs only.
To preserve current behavior all controllers are auto attached.
To not not auto attach controllers the nvmf_subsystem_add_ns
shall be called with "--no-auto-attach". We introduce two new
RPC calls:
- nvmf_ns_attach_ctrlr <subsysNQN> <NSID> [--host <hostNQN>]
- nvmf_ns_detach_ctrlr <subsysNQN> <NSID> [--host <hostNQN>]
If no host NQN is specified all controllers
(new and currently connected) will attach/detach to/from the
namespace specified.
The list in spdk_nvmf_ns is used to keep track of hostNQNs
which controllers should be attached on connect.
The active_ns array in spdk_nvmf_ctrlr is used for fast lookup
to check whether a NSID is active/inactive on command execution.

Thanks,
Jonas