eventfs: Fix use-after-free in eventfs_remove_rec()

In the Linux kernel, the following vulnerability has been resolved: eventfs: Fix use-after-free in eventfs_remove_rec() eventfs_remove_rec() recursively removes the child at the current loop position. After the recursive call returns, list_for_each_entry() advances by reading list.next from the removed child. If free_ei() drops the final reference, release_ei() reuses the list/rcu union to queue an SRCU callback. The child may be freed before that read. The eventfs_mutex serializes list updates, but it does not keep the removed child alive or prevent the SRCU callback from running. Use list_for_each_entry_safe() to save the next sibling before recursively removing the current child.