ethtool: Clarify len/n_stats fields in/out semantics

ethtool: Clarify len/n_stats fields in/out semantics

Document that the 'len' field in ethtool_gstrings and 'n_stats' field in
ethtool_stats optionally serve dual purposes: on entry they specify the
number of items requested, and on return they indicate the number
actually returned (which is not necessarily the same).

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Link: https://patch.msgid.link/20260115060544.481550-1-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>