dcache stuff for this cycle

change of locking rules for __dentry_kill(), regularized refcounting
rules in that area, assorted cleanups and removal of weird corner
cases (e.g. now ->d_iput() on child is always called before the parent
might hit __dentry_kill(), etc.)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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