nvmet: nul-terminate the NQNs passed in the connect command

[ Upstream commit 1c22e0295a5eb571c27b53c7371f95699ef705ff ]

The host and subsystem NQNs are passed in the connect command payload and
interpreted as nul-terminated strings.  Ensure they actually are
nul-terminated before using them.

Fixes: a07b4970 "nvmet: add a generic NVMe target")
Reported-by: Alon Zahavi <zahavi.alon@gmail.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>