netlink: cmp: shift rhs constant if lhs offset doesn't start on byte boundary

netlink: cmp: shift rhs constant if lhs offset doesn't start on byte boundary

if we have payload(someoffset) == 42, then shift 42 in case someoffset
doesn't start on a byte boundary.

We already insert a mask instruction to only load those bits into
the register that we were interested in, but the cmp will fail without
also adjusting rhs accordingly.

Needs additional patch in reverse direction to undo the shift again
when dumping ruleset.

Signed-off-by: Florian Westphal <fw@strlen.de>