fs: eventfd: fix identation on kernel-doc

Sphinx require explicit tags in order to use a list of possible
values, otherwise it produces this error:

	./fs/eventfd.c:219: WARNING: Option list ends without a blank line; unexpected unindent.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

fs/eventfd.c

@@ -215,8 +215,8 @@ EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
  *
  * Returns %0 if successful, or the following error codes:
  *
- * -EAGAIN      : The operation would have blocked but @no_wait was non-zero.
- * -ERESTARTSYS : A signal interrupted the wait operation.
+ *  - -EAGAIN      : The operation would have blocked but @no_wait was non-zero.
+ *  - -ERESTARTSYS : A signal interrupted the wait operation.
  *
  * If @no_wait is zero, the function might sleep until the eventfd internal
  * counter becomes greater than zero.