fs: handle failed audit_log_start properly

audit_log_start() may return NULL, this is unchecked by the caller in
audit_log_link_denied() and could cause a NULL ptr deref.

Introduced by commit a51d9eaa ("fs: add link restriction audit reporting").
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/audit.c

@@ -1440,6 +1440,8 @@ void audit_log_link_denied(const char *operation, struct path *link)
 
 	ab = audit_log_start(current->audit_context, GFP_KERNEL,
 			     AUDIT_ANOM_LINK);
+	if (!ab)
+		return;
 	audit_log_format(ab, "op=%s action=denied", operation);
 	audit_log_format(ab, " pid=%d comm=", current->pid);
 	audit_log_untrustedstring(ab, current->comm);