Binary operator expressions where the left hand-side expression is a
reference to the packet are replaced by a call to the bpf_dins_pkt
helper. When replacing text, the Clang Rewriter tries to maintain a
list of offsets between the original and the new position of tokens.

Replacing the whole binary operator expression with the call to
bpf_dins_pkt confuses the Rewriter and it is unable to track the new
position of the right hand-side expression. Rewriting the binary
operator expression in two times without rewriting the right
hand-side expression itself solves the issue.