ethtool: work-around ETHTOOL_GRSSH/ETHTOOL_SRSSH ABI breakage

ethtool: work-around ETHTOOL_GRSSH/ETHTOOL_SRSSH ABI breakage

ethtool since version 6.9 introduced support for getting/setting RSS
input transformation supported in Linux since version 6.8.

The now changed kernel ioctl ABI, however, cannot be detected from
userland, and ethtool since version 6.9 simply assumes that a previously
reserved field is now used to set the input transformation.
Unfortunately the default value RXH_XFRM_NO_CHANGE (0xff) used by ethtool
userland creates an incompatibility with older kernels which cannot be
resolved easily without introducing even more ABI breakage.

Work-around the issue and fix support for --set-rxfh and --set-rxfh-indir
ethtool userland tool commands by making the support for input_xfrm
conditional on compile time, and keep it disabled for Linux 6.6.

Fixes: 8c2dcd1518 ("ethtool: update to 6.10")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>