client: fix race in concurrent readdir

For a large directory, program needs to issue multiple readdir
syscalls to get all dentries. When there are multiple programs
read the directory concurrently. Following sequence of events
can happen.

 - program calls readdir with pos = 2. ceph sends readdir request
   to mds. The reply contains N1 entries. ceph adds these N1 entries
   to readdir cache.
 - program calls readdir with pos = N1+2. The readdir is satisfied
   by the readdir cache, N2 entries are returned. (Other program
   calls readdir in the middle, which fills the cache)
 - program calls readdir with pos = N1+N2+2. ceph sends readdir
   request to mds. The reply contains N3 entries and it reaches
   directory end. ceph adds these N3 entries to the readdir cache
   and marks directory complete.

The second readdir call does not update dirp->cache_index. ceph adds
the last N3 entries to wrong places.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Fixes: http://tracker.ceph.com/issues/23894

src/client/Client.cc

@@ -7717,6 +7717,7 @@ int Client::_readdir_cache_cb(dir_result_t *dirp, add_dirent_cb_t cb, void *p,
     else
       dirp->next_offset = dirp->offset_low();
     dirp->last_name = dn_name; // we successfully returned this one; update!
+    dirp->release_count = 0; // last_name no longer match cache index
     if (r > 0)
       return r;
   }