nvmet: implement basic In-Band Authentication

Implement NVMe-oF In-Band authentication according to NVMe TPAR 8006.
This patch adds three additional configfs entries 'dhchap_key',
'dhchap_ctrl_key', and 'dhchap_hash' to the 'host' configfs directory.
The 'dhchap_key' and 'dhchap_ctrl_key' entries need to be in the ASCII
format as specified in NVMe Base Specification v2.0 section 8.13.5.8
'Secret representation'.
'dhchap_hash' defaults to 'hmac(sha256)', and can be written to to
switch to a different HMAC algorithm.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>