nvmet: Improve nvmet_alloc_ctrl() interface and implementation

Introduce struct nvmet_alloc_ctrl_args to define the arguments for
the function nvmet_alloc_ctrl() to avoid the need for passing a pointer
to a struct nvmet_req as an argument. This new data structure aggregates
together the arguments that were passed to nvmet_alloc_ctrl()
(subsysnqn, hostnqn and kato), together with the struct nvmet_req fields
used by nvmet_alloc_ctrl(), that is, the fields port, p2p_client, and
ops as input and the result and error_loc fields as output, as well as a
status field. nvmet_alloc_ctrl() is also changed to return a pointer
to the allocated and initialized controller structure instead of a
status code, as the status is now returned through the status field of
struct nvmet_alloc_ctrl_args.

The function nvmet_setup_p2p_ns_map() is changed to not take a pointer
to a struct nvmet_req as argument, instead, directly specify the
p2p_client device pointer needed as argument.

The code in nvmet_execute_admin_connect() that initializes a new target
controller after allocating it is moved into nvmet_alloc_ctrl().
The code that sets up an admin queue for the controller (and the call
to nvmet_install_queue()) remains in nvmet_execute_admin_connect().

Finally, nvmet_alloc_ctrl() is also exported to allow target drivers to
use this function directly to allocate and initialize a new controller
structure without the need to rely on a fabrics connect command request.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Rick Wertenbroek <rick.wertenbroek@gmail.com>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>