ksmbd: lock the binding preauth session in smb3_preauth_hash_rsp

ksmbd: lock the binding preauth session in smb3_preauth_hash_rsp

smb3_preauth_hash_rsp() computes the SMB3.1.1 preauth integrity hash on
the response path. For a binding SESSION_SETUP it looks up the
per-connection preauth_session and reads its Preauth_HashValue.

smb2_sess_setup() frees that preauth_session under ksmbd_conn_lock().
Two SMB2 requests on one connection can run concurrently, so an unlocked
lookup and hash can use a preauth_session after another worker frees it.

Take ksmbd_conn_lock() before selecting conn->binding and hold it across
the selected preauth hash lookup and update. This preserves the existing
hash selection while preventing the lookup-to-use lifetime race.

Fixes: 1c5daa2ea924 ("ksmbd: handle channel binding with a different user")
Signed-off-by: Gil Portnoy <dddhkts1@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>