fixing rename, take two

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

Rename is the hairiest vop and has the insanest API of them all, and
in most of our file systems it's hosed.  I'd like to fix that.

A few months ago I asked here for review on a patch to implement a
generic abstraction with a saner protocol for implementing rename, and
to make several file systems use this.  That way, we can eventually
make them all use the generic abstraction and replace the current
insane VOP_RENAME protocol.

But maybe it was too large a patch to review.  So I've found a bit of
time to update the patch, simplify some of the changes, and split it
into a few smaller patches (with apologies to the mailing list
archive):

http://www.NetBSD.org/~riastradh/patches/rename/genfs_rename.patch
  implements the generic abstraction, genfs_rename, which handles the
  hairy common logic of ancestry checks, lock order, permissions, and
  so on;

http://www.NetBSD.org/~riastradh/patches/rename/tmpfs_genfs_rename.patch
  makes tmpfs use genfs_rename;

http://www.NetBSD.org/~riastradh/patches/rename/ufs_genfs_rename.patch
  makes ffs, lfs, and ext2fs use genfs_rename; and

http://www.NetBSD.org/~riastradh/patches/rename/t_renamerace.patch
http://www.NetBSD.org/~riastradh/patches/rename/fstest_fix.patch
http://www.NetBSD.org/~riastradh/patches/rename/rename_tests.patch
  all update, fix, and add tests related to rename.

With these patches, tmpfs, ffs, lfs, and ext2fs all pass all the tests
I've thrown at them.  (Previously ext2fs did not, and there was a lot
of hairy logic duplicated between tmpfs, ffs, and lfs.)

I would like to commit at least the first three (genfs_rename.patch,
tmpfs_genfs_rename.patch, and ufs_genfs_rename.patch), but I don't
want to do so unless they've been reviewed by someone else.  Please
take a look and tell me what I've done horribly wrong and how this is
going to hose NetBSD forever if I even think about committing any of
it.