nvme-loop: initialize sqsize later

Defer initializing the sqsize field from the options until it has been
capped by MAXCMD.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>