union mounts via ->get_unlinked_inode()

[Valerie Aurora <vaurora@redhat.com>]

Hey Miklos,

At the FS workshop, I came up with an idea that will fix the
crash-during-copyup bug and also allow atomic creation of
whiteouts/fallthrus/opaque flags using xattrs, as you proposed.

Instead of ->whiteout() and ->fallthru(), we add just one new file
system op, ->get_unlinked_inode(), which returns an inode that is
allocated but not linked into the namespace - basically, ->create()
followed by ->unlink() without a window in which the partly
initialized file will be visible if the system crashes.  This is
usually already implemented internally by the file system, just not
exported, so it should be minimally invasive.  Then we can copy up
data, set owner, set the whiteout xattr, etc. and when the inode is
consistent, link it into the namespace.  Currently, whiteouts and
fallthrus are implemented with an in-dentry flag so there is no
crash-atomicity problem, but there will be with the xattr-based
version.

So, if the ->get_unlinked_inode() based version works out, I would be
very interested in seeing your whiteout/opaque/fallthru via xattrs
implementation and merging it into my tree.

Thanks,

-VAL