nvme-auth: use hkdf_expand_label()

When generating keying material during an authentication transaction
(secure channel concatenation), the HKDF-Expand-Label function is part
of the specified key derivation process.

The current open-coded implementation misses the length prefix
requirements on the HkdfLabel label and context variable-length vectors
(RFC 8446 Section 3.4).

Instead, use the hkdf_expand_label() function.

Signed-off-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>