client: add assert to catch dentry ref drop during unlink

Add ceph_assert(dn->ref > 0) after Dentry::unlink() in Client::unlink()
to catch the case where internal put() calls drop the dentry reference
to zero before we proceed to detach/lru_remove.

This turns a silent use-after-free into a clean assertion failure with
a full stack trace, making it possible to catch the exact conditions
in a test environment.

Signed-off-by: Igor Golikov <igolikov@ibm.com>
Fixes: https://tracker.ceph.com/issues/74625