Fix a bug whereby a user with write access to a directory being removed

Fix a bug whereby a user with write access to a directory being removed
could cause the removal of that directory to fail with an erroneous
diagnostic about a directory cycle.  Reported by Vineet Chadha.

(AD_pop_and_chdir): If the directory we're about to
leave (and try to rmdir) is the one whose dev_ino is being used to
detect a cycle, reset cycle_check_state.dev_ino to that of the parent.