nvmet: cq: prepare for completion queue sharing

nvmet: cq: prepare for completion queue sharing

For the PCI transport, the NVMe specification allows submission queues
to share completion queues, however, this is not supported in the
current NVMe target implementation. This is a preparatory patch to allow
for completion queue (CQ) sharing between different submission queues
(SQ).

To support queue sharing, reference counting completion queues is
required. This patch adds the refcount_t field ref to struct nvmet_cq
coupled with respective nvmet_cq_init(), nvmet_cq_get(), nvmet_cq_put(),
nvmet_cq_is_deletable() and nvmet_cq_destroy() functions.

A CQ reference count is initialized with nvmet_cq_init() when a CQ is
created. Using nvmet_cq_get(), a reference to a CQ is taken when an SQ is
created that uses the respective CQ. Similarly. when an SQ is destroyed,
the reference count to the respective CQ from the SQ being destroyed is
decremented with nvmet_cq_put(). The last reference to a CQ is dropped
on a CQ deletion using nvmet_cq_put(), which invokes nvmet_cq_destroy()
to fully cleanup after the CQ. The helper function nvmet_cq_in_use() is
used to determine if any SQs are still using the CQ pending deletion.
In which case, the CQ must not be deleted. This should protect scenarios
where a bad host may attempt to delete a CQ without first having deleted
SQ(s) using that CQ.

Additionally, this patch adds an array of struct nvmet_cq to the
nvmet_ctrl structure. This allows for the controller to keep track of CQs
as they are created and destroyed, similar to the current tracking done
for SQs. The memory for this array is freed when the controller is freed.
A struct nvmet_ctrl reference is also added to the nvmet_cq structure to
allow for CQs to be removed from the controller whilst keeping the new
API similar to the existing API for SQs.

Sample callchain with CQ refcounting for the PCI endpoint target
(pci-epf):

i.   nvmet_execute_create_cq -> nvmet_pci_epf_create_cq
     -> nvmet_cq_create -> nvmet_cq_init [cq refcount=1]

ii.  nvmet_execute_create_sq -> nvmet_pci_epf_create_sq
     -> nvmet_sq_create -> nvmet_sq_init -> nvmet_cq_get [cq refcount=2]

iii. nvmet_execute_delete_sq - > nvmet_pci_epf_delete_sq ->
     -> nvmet_sq_destroy -> nvmet_cq_put [cq refcount 1]

iv.  nvmet_execute_delete_cq -> nvmet_pci_epf_delete_cq
     -> nvmet_cq_put [cq refcount 0]

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>