sfc: fix field-spanning memcpy in selftest

Add a struct_group for the whole packet body so we can copy it in one
 go without triggering FORTIFY_SOURCE complaints.

Fixes: cf60ed46 ("sfc: use padding to fix alignment in loopback test")
Fixes: 30c24dd8 ("sfc: siena: use padding to fix alignment in loopback test")
Fixes: 1186c6b3

 ("sfc: falcon: use padding to fix alignment in loopback test")
Reviewed-by: Andy Moreton <andy.moreton@amd.com>
Tested-by: Andy Moreton <andy.moreton@amd.com>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230728165528.59070-1-edward.cree@amd.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>