client: crash caused by invalid iterator in _readdir_cache_cb

client: crash caused by invalid iterator in _readdir_cache_cb

Capacity of `readdir_cache` may change after `client_lock` is unlocked in iterations of `readdir_cache`,
and it can cause the iterator to be invalid, then using the invalid iterator in the next iteration will cause crash.
Crash may happen at `Dentry *dn = *pd` (pd points to invalid memory),
or at `if (pd >= dir->readdir_cache.end() || *pd != dn)` (pd is smaller than begin() if idx is negative).
Use index instead of iterator to solve this problem.

Fixes: https://tracker.ceph.com/issues/72247
Signed-off-by: Zhansong Gao <zhsgao@hotmail.com>
(cherry picked from commit 9e0488dd0bda18ab2abd96cdb3ec18034c1e85f1)