Fix a tiny race in tdb_delete() between TDBF_DELETED, tdb_unlink()

Fix a tiny race in tdb_delete() between TDBF_DELETED, tdb_unlink()
and tdb_cleanspd().  gettdb...() can return a TDB before tdb_unlink().
Then ipsp_spd_lookup() could add it to tdb_policy_head after
tdb_cleanspd().  There it would stay until it hits the kassert in
tdb_free().
OK tobhe@