bftw: Make sure we don't close a directory while we unwrap it

bftw: Make sure we don't close a directory while we unwrap it

bftw_cache_reserve() can lead to bftw_cache_pop(), which could close the
directory we're trying to unwrap!  If that happened, we would try
dup_cloexec(-1), which would fail with EBADF, so there was no observable
bug.  But it's better to avoid the whole situation.