tc: u32: Fix key folding in sample option

tc: u32: Fix key folding in sample option

In between Linux kernel 2.4 and 2.6, key folding for hash tables changed
in kernel space. When iproute2 dropped support for the older algorithm,
the wrong code was removed and kernel 2.4 folding method remained in
place. To get things functional for recent kernels again, restoring the
old code alone was not sufficient - additional byteorder fixes were
needed.

While being at it, make use of ffs() and thereby align the code with how
kernel determines the shift width.

Fixes: 267480f55383c ("Backout the 2.4 utsname hash patch.")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>