Linuxmodule clickfs: Revalidate directory entries.

The clickfs-01.testie test failed sporadically. Investigation showed
that, at least on recent Linuxes (3.5), /click dcache entries for
previous configuration generations were sticking around and polluting
the cache. Should we call d_genocide for each superblock? Unclear; but
it does work if we add a true d_revalidate callback and revalidate the
dentry on use. This in turn requires updates to proclikefs, or a
handler file held open across module unload causes a crash.

I'm sure these updates to clickfs/proclikefs break prior linuxes, we
will wait to hear from users.

Signed-off-by: Eddie Kohler <ekohler@gmail.com>