vfs: move dentry to its new location during file rename

If dentry has elevated reference count during rename (a file is opened
for instance) it is not destroyed and still hashed with old path. As a
result renamed file can still be accessed by its old name.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>