ethtool: Fix and simplify ethtool_convert_link_mode_to_legacy_u32()

Fix the implementation of ethtool_convert_link_mode_to_legacy_u32(), which
is supposed to return false if src has bits higher than 31 set. The current
implementation uses the complement of bitmap_fill(ext, 32) to test high
bits of src, which is wrong as bitmap_fill() fills _with long granularity_,
and sizeof(long) can be > 4. No users of this function currently check the
return value, so the bug was dormant.

Also remove the check for __ETHTOOL_LINK_MODE_MASK_NBITS > 32, as the enum
ethtool_link_mode_bit_indices contains far beyond 32 values. Using
find_next_bit() to test the src bitmask works regardless of this anyway.

Signed-off-by: Marco Bonelli <marco@mebeim.net>
Link: https://lore.kernel.org/r/20220609134900.11201-1-marco@mebeim.net


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