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>