CVE-2026-72318: cifs: validate DFS referral string offsets

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

cifs: validate DFS referral string offsets

parsedfsreferrals() validates that the response header and referral array fit in the received buffer, but each referral also contains string offsets supplied by the server.

Those offsets are used to compute the DfsPath and NetworkAddress string pointers without checking whether they still point inside the response buffer. A malformed referral can therefore make the computed pointer exceed the end of the buffer. The resulting negative maxlen is then passed to cifsstrndupfromutf16(), and the non-Unicode path forwards it to kstrndup() as a sizet, allowing strnlen() to read out of bounds.

Validate each string offset before deriving the string pointer.