nvmet: lock config semaphore when accessing DH-HMAC-CHAP key

When the DH-HMAC-CHAP key is accessed via configfs we need to take the
config semaphore as a reconnect might be running at the same time.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>