cifs: fix race in assemble_neg_contexts()

jira VULN-169227
cve CVE-2023-53751
commit-author Paulo Alcantara <pc@cjr.nz>
commit 775e44d
upstream-diff Ignored the introduction of `pserver' variable, as well as
  the usage of `hostname' local, as they were only needed in the upstream
  because of the dual source of the server hostname, introduced in the
  non-backported commit 9de7499 ("smb3:
  use netname when available on secondary channels")

Serialise access of TCP_Server_Info::hostname in
assemble_neg_contexts() by holding the server's mutex otherwise it
might end up accessing an already-freed hostname pointer from
cifs_reconnect() or cifs_resolve_server().

	Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
	Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
	Signed-off-by: Steve French <stfrench@microsoft.com>
(cherry picked from commit 775e44d)
	Signed-off-by: Marcin Wcisło <marcin.wcislo@conclusive.pl>

Author: pvts-mat

fs/cifs/smb2pdu.c

@@ -549,8 +549,10 @@ assemble_neg_contexts(struct smb2_negotiate_req *req,
 	} else
 		req->NegotiateContextCount = cpu_to_le16(4);
 
+	cifs_server_lock(server);
 	ctxt_len = build_netname_ctxt((struct smb2_netname_neg_context *)pneg_ctxt,
 					server->hostname);
+	cifs_server_unlock(server);
 	*total_len += ctxt_len;
 	pneg_ctxt += ctxt_len;