nvme-fabrics: check max outstanding commands

Maxcmd is mandatory for fabrics, check it early to identify the root
cause instead of waiting for it to propagate to "sqsize" and "allocing
queue".

By the way, change nvme_check_ctrl_fabric_info() to
nvmf_validate_identify_ctrl().

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>