CVE-2026-31478

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

ksmbd: replace hardcoded hdr2_len with offsetof() in smb2_calc_max_out_buf_len()

After this commit (e2b76ab8b5c9 "ksmbd: add support for read compound"),
response buffer management was changed to use dynamic iov array.
In the new design, smb2_calc_max_out_buf_len() expects the second
argument (hdr2_len) to be the offset of ->Buffer field in the
response structure, not a hardcoded magic number.
Fix the remaining call sites to use the correct offsetof() value.