nvmet: pci-epf: Keep completion queues mapped

Instead of mapping and unmapping the completion queues memory to the
host PCI address space whenever nvmet_pci_epf_cq_work() is called, map
a completion queue to the host PCI address space when the completion
queue is created with nvmet_pci_epf_create_cq() and unmap it when the
completion queue is deleted with nvmet_pci_epf_delete_cq().

This removes the completion queue mapping/unmapping from
nvmet_pci_epf_cq_work() and significantly increases performance. For
a single job 4K random read QD=1 workload, the IOPS is increased from
23 KIOPS to 25 KIOPS. Some significant throughput increasde for high
queue depth and large IOs workloads can also be seen.

Since the functions nvmet_pci_epf_map_queue() and
nvmet_pci_epf_unmap_queue() are called respectively only from
nvmet_pci_epf_create_cq() and nvmet_pci_epf_delete_cq(), these functions
are removed and open-coded in their respective call sites.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>

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