ice: Mark extack argument as __always_unused

Commit 87b0984e ("net: Add extack argument to ndo_fdb_add()") in
net-next added an extended parameter to the .ndo_fdb_add op and changed
ice_fdb_add() accordingly. Update the function header and add the
__always_unused attribute to the new parameter to avoid -Wunused-parameter
warnings.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>