nvmet: Don't overflow subsysnqn

nvmet_root_discovery_nqn_store treats the subsysnqn string like a fixed
size buffer, even though it is dynamically allocated to the size of the
string.

Create a new string with kstrndup instead of using the old buffer.

Reported-by:  <syzbot+ff4aab278fa7e27e0f9e@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=ff4aab278fa7e27e0f9e


Fixes: 95409e27 ("nvmet: implement unique discovery NQN")
Signed-off-by: Leo Stone <leocstone@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>