nvme-pci: move use_sgl initialization to nvme_init_iod()

A flag "use_sgl" of "struct nvme_iod" has been used in nvme_init_iod()
without being set to any value. It seems like "use_sgl" has been set
in either nvme_pci_setup_prps() or nvme_pci_setup_sgls() which occur
later than nvme_init_iod().

Make "iod->use_sgl" being set in a proper place, nvme_init_iod().
Also move nvme_pci_use_sgls() up above nvme_init_iod() to make it
possible to be called by nvme_init_iod().

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>