net/mlx5: Added missing check of msg length in verifying its signature

Set and verify signature calculates the signature for each of the
mailbox nodes, even for those that are unused (from cache). Added
a missing length check to set and verify only those which are used.

While here, also moved the setting of msg's nodes token to where we
already go over them. This saves a pass because checksum is disabled,
and the only useful thing remaining that set signature does is setting
the token.

Fixes: e126ba97 ('mlx5: Add driver for Mellanox Connect-IB
adapters')
Signed-off-by: Paul Blakey <paulb@mellanox.com>

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>