src: Optimize prefix matches on byte-boundaries

src: Optimize prefix matches on byte-boundaries

If a prefix expression's length is on a byte-boundary, it is sufficient
to just reduce the length passed to "cmp" expression. No need for
explicit bitwise modification of data on LHS. The relevant code is
already there, used for string prefix matches. There is one exception
though, namely zero-length prefixes: Kernel doesn't accept zero-length
"cmp" expressions, so keep them in the old code-path for now.

This patch depends upon the previous one to correctly parse odd-sized
payload matches but has to extend support for non-payload LHS as well.
In practice, this is needed for "ct" expressions as they allow matching
against IP address prefixes, too.

Signed-off-by: Phil Sutter <phil@nwl.cc>