nvme-fc: avoid null pointer dereference

Before using dynamically allcoated variable lsop in the
nvme_fc_rcv_ls_req(), add a check for NULL and error out early.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>