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

net/ethtool/ioctl.c

@@ -369,22 +369,9 @@ EXPORT_SYMBOL(ethtool_convert_legacy_u32_to_link_mode);
 bool ethtool_convert_link_mode_to_legacy_u32(u32 *legacy_u32,
 					     const unsigned long *src)
 {
-	bool retval = true;
-
-	/* TODO: following test will soon always be true */
-	if (__ETHTOOL_LINK_MODE_MASK_NBITS > 32) {
-		__ETHTOOL_DECLARE_LINK_MODE_MASK(ext);
-
-		linkmode_zero(ext);
-		bitmap_fill(ext, 32);
-		bitmap_complement(ext, ext, __ETHTOOL_LINK_MODE_MASK_NBITS);
-		if (linkmode_intersects(ext, src)) {
-			/* src mask goes beyond bit 31 */
-			retval = false;
-		}
-	}
 	*legacy_u32 = src[0];
-	return retval;
+	return find_next_bit(src, __ETHTOOL_LINK_MODE_MASK_NBITS, 32) ==
+		__ETHTOOL_LINK_MODE_MASK_NBITS;
 }
 EXPORT_SYMBOL(ethtool_convert_link_mode_to_legacy_u32);