net: deal with fragment-overlapping-two-holes case

With a suitable sequence of malicious packets, it's currently possible
to get a hole descriptor to contain arbitrary attacker-controlled
contents, and then with one more packet to use that as an arbitrary
write vector.

While one could possibly change the algorithm so we instead loop over
all holes, and in each hole puts as much of the current fragment as
belongs there (taking care to carefully update the hole list as
appropriate), it's not worth the complexity: In real, non-malicious
scenarios, one never gets overlapping fragments, and certainly not
fragments that would be supersets of one another.

So instead opt for this simple protection: Simply don't allow the
eventual memcpy() to write beyond the last_byte of the current hole.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>