CVE-2026-53366: ipv4: account for fraggap on the paged allocation path

In the Linux kernel, the following vulnerability has been resolved:

ipv4: account for fraggap on the paged allocation path

In ipappenddata(), when the paged-allocation branch is taken, alloclen and pagedlen are computed as

alloclen = fragheaderlen + transhdrlen; pagedlen = datalen - transhdrlen;

datalen already includes fraggap, but the fraggap bytes carried over from the previous skb are copied into the new skb's linear area at offset transhdrlen by the subsequent skbcopyandcsumbits(). The linear area is therefore undersized by fraggap bytes while pagedlen is overstated by the same amount.

The non-paged branch sets alloclen to fraglen, which already accounts for fraggap because datalen does. Bring the paged branch in line by adding fraggap to alloclen and subtracting it from pagedlen.

After this adjustment, copy no longer collapses to -fraggap on the paged path, so remove the stale comment describing that old arithmetic.