idpf: avoid compiler introduced padding in virtchnl2_rss_key struct

Size of the virtchnl2_rss_key struct should be 7 bytes but the
compiler introduces a padding byte for the structure alignment.
This results in idpf sending an additional byte of memory to the device
control plane than the expected buffer size. As the control plane
enforces virtchnl message size checks to validate the message,
set RSS key message fails resulting in the driver load failure.

Remove implicit compiler padding by using "__packed" structure
attribute for the virtchnl2_rss_key struct.

Also there is no need to use __DECLARE_FLEX_ARRAY macro for the
'key_flex' struct field. So drop it.

Fixes: 0d7502a9 ("virtchnl: add virtchnl version 2 ops")
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Signed-off-by: Pavan Kumar Linga <pavan.kumar.linga@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Scott Register <scott.register@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>