nvmet: add module description to stop warnings

Add MODULE_DESCRIPTION() in order to remove warnings & get clean build:-

WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/nvme/target/nvmet.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/nvme/target/nvme-loop.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/nvme/target/nvmet-rdma.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/nvme/target/nvmet-fc.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/nvme/target/nvme-fcloop.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/nvme/target/nvmet-tcp.o

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