dm-integrity: fix non-constant-time tag verification

When using dm-integrity in standalone mode with a keyed hmac algorithm,
integrity tags are calculated and verified internally.

Using plain memcmp to compare the stored and computed tags may leak the
position of the first byte mismatch through side-channel analysis,
allowing to brute-force expected tags in linear time (e.g., by counting
single-stepping interrupts in confidential virtual machine environments).

Co-developed-by: Luca Wilke <work@luca-wilke.com>
Signed-off-by: Luca Wilke <work@luca-wilke.com>
Signed-off-by: Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org