nvme-tcp: fix queue mapping when queue count is limited

When the controller supports less queues than requested, we
should make sure that queue mapping does the right thing and
not assume that all queues are available. This fixes a crash
when the controller supports less queues than requested.

The rules are:
1. if no write queues are requested, we assign the available queues
   to the default queue map. The default and read queue maps share the
   existing queues.
2. if write queues are requested:
  - first make sure that read queue map gets the requested
    nr_io_queues count
  - then grant the default queue map the minimum between the requested
    nr_write_queues and the remaining queues. If there are no available
    queues to dedicate to the default queue map, fallback to (1) and
    share all the queues in the existing queue map.

Also, provide a log indication on how we constructed the different
queue maps.

Reported-by: Harris, James R <james.r.harris@intel.com>
Tested-by: Jim Harris <james.r.harris@intel.com>
Cc: <stable@vger.kernel.org> # v5.0+
Suggested-by: Roy Shterman <roys@lightbitslabs.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>