ethtool: Track user-provided RSS indirection table size

ethtool: Track user-provided RSS indirection table size

Track the number of indirection table entries the user originally
provided (context 0/default as well!).

Replace IFF_RXFH_CONFIGURED with rss_indir_user_size: the flag is
redundant now that user_size captures the same information.

Add ethtool_rxfh_indir_lost() for drivers that must reset the
indirection table.

Convert bnxt and mlx5 to use it.

Signed-off-by: Björn Töpel <bjorn@kernel.org>
Link: https://patch.msgid.link/20260320085826.1957255-2-bjorn@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>