ksmbd: reject repeated SMB2 NEGOTIATE requests

Summary

In the Linux kernel, the following vulnerability has been resolved: ksmbd: reject repeated SMB2 NEGOTIATE requests Unauthenticated client can send multiple successful SMB2 NEGOTIATE requests on one connection before SESSION_SETUP. While the connection is in KSMBD_SESS_NEED_SETUP, smb2_handle_negotiate() accepts another SMB3.1.1 NEGOTIATE and overwrites conn->preauth_info with a new allocation. Only the final allocation is freed when the connection is released, leaking one object for every additional successful request. A repeated SMB2 NEGOTIATE after a dialect has been selected is a protocol violation. MS-SMB2 section 3.3.5.4 requires the server to disconnect without replying in this case. Set the connection exiting when rejecting the request, in addition to suppressing the response. Reject SMB2 NEGOTIATE unless the connection is new or is waiting for the SMB2 NEGOTIATE that follows an SMB1 multi-protocol negotiate. Serialize both SMB1 and SMB2 negotiation paths under conn->srv_mutex, since they update connection-wide dialect and negotiation state. Move the locking contract to ksmbd_smb_negotiate_common(), where the state and dialect are selected, and add ksmbd_conn_new() for consistent state access.