namei: Do not remove n_voldir1

namei: Do not remove n_voldir1

When removing data directories in namei_RemoveDataDirectories, do not
remove the n_voldir1 directory (directory X in /vicepa/AFSIDat/X).
Removing this directory can race against the creation of an entirely
unrelated volume, causing the create op to fail (since it tries to
create a directory in a directory that no longer exists).

We don't currently have the necessary locking to make this safe, and
since the overhead of n_voldir1 existing is pretty negligible, just
leave it there. Also add some comments briefly justifying this.

Note that other similar races probably exist for directories under
n_voldir1, but they would only be between volumes in the same VG, and
so are much less likely to occur.

Change-Id: I5eac894cdbdb3fd1d74d8a0fc226df03686ceeef
Reviewed-on: http://gerrit.openafs.org/2727
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>