ksmbd: fix heap out-of-bounds write in krb5_authenticate()

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

ksmbd: fix heap out-of-bounds write in krb5_authenticate()

In krb5_authenticate(), out_len is calculated to determine available
headroom in response_buf for the incoming Kerberos AP-REP token. However,
when SMB2_SESSION_SETUP is processed as a non-first element of a compounded
SMB2 request, the calculation omits work->next_smb2_rsp_hdr_off.

This causes out_len to overstate remaining buffer headroom by
the cumulative size of prior responses in the compound chain. Consequently,
the length check in ksmbd_krb5_authenticate()
(*out_len <= resp->spnego_blob_len) passes erroneously, allowing memcpy()
to write the AP-REP blob past the end of response_buf into adjacent kernel
heap memory.

Fix this by subtracting work->next_smb2_rsp_hdr_off when computing out_len,
ensuring it accurately reflects physical remaining buffer space.