audit: don't WARN_ON_ONCE(!current->mm) in audit_exe_compare()

eBPF can end up calling into the audit code from some odd places, and
some of these places don't have @current set properly so we end up
tripping the `WARN_ON_ONCE(!current->mm)` near the top of
`audit_exe_compare()`.  While the basic `!current->mm` check is good,
the `WARN_ON_ONCE()` results in some scary console messages so let's
drop that and just do the regular `!current->mm` check to avoid
problems.

Cc: <stable@vger.kernel.org>
Fixes: 47846d5 ("audit: don't take task_lock() in audit_exe_compare() code path")
Reported-by: Artem Savkov <asavkov@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit_watch.c

@@ -531,7 +531,7 @@ int audit_exe_compare(struct task_struct *tsk, struct audit_fsnotify_mark *mark)
 	if (tsk != current)
 		return 0;
 
-	if (WARN_ON_ONCE(!current->mm))
+	if (!current->mm)
 		return 0;
 	exe_file = get_mm_exe_file(current->mm);
 	if (!exe_file)