saner calling conventions for ->d_automount()

Currently the calling conventions for ->d_automount() instances have
an odd wart - returned new mount to be attached is expected to have
refcount 2.

That kludge is intended to make sure that mark_mounts_for_expiry() called
before we get around to attaching that new mount to the tree won't decide
to take it out.  finish_automount() drops the extra reference after it's
done with attaching mount to the tree - or drops the reference twice in
case of error.  ->d_automount() instances have rather counterintuitive
boilerplate in them.

There's a much simpler approach: have mark_mounts_for_expiry() skip the
mounts that are yet to be mounted.  And to hell with grabbing/dropping
those extra references.  Makes for simpler correctness analysis, at that...

Reviewed-by: Christian Brauner <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Reviewed-by: Paulo Alcantara (Red Hat) <[email protected]>
Acked-by: David Howells <[email protected]>
Tested-by: David Howells <[email protected]>
Acked-by: Steven Rostedt (Google) <[email protected]>
Signed-off-by: Al Viro <[email protected]>

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