list_lru: remove special case function list_lru_dispose_all.

The list_lru implementation has one function, list_lru_dispose_all, with
only one user (the dentry code).  At first, such function appears to make
sense because we are really not interested in the result of isolating each
dentry separately - all of them are going away anyway.  However, it's
implementation is buggy in the following way:

When we call list_lru_dispose_all in fs/dcache.c, we scan all dentries
marking them with DCACHE_SHRINK_LIST.  However, this is done without the
nlru->lock taken.  The imediate result of that is that someone else may
add or remove the dentry from the LRU at the same time.  When list_lru_del
happens in that scenario we will see an element that is not yet marked
with DCACHE_SHRINK_LIST (even though it will be in the future) and
obviously remove it from an lru where the element no longer is.  Since
list_lru_dispose_all will in effect count down nlru's nr_items and
list_lru_del will do the same, this will lead to an imbalance.

The solution for this would not be so simple: we can obviously just keep
the lru_lock taken, but then we have no guarantees that we will be able to
acquire the dentry lock (dentry->d_lock).  To properly solve this, we need
a communication mechanism between the lru and dentry code, so they can
coordinate this with each other.

Such mechanism already exists in the form of the list_lru_walk_cb
callback.  So it is possible to construct a dcache-side prune function
that does the right thing only by calling list_lru_walk in a loop until no
more dentries are available.

With only one user, plus the fact that a sane solution for the problem
would involve boucing between dcache and list_lru anyway, I see little
justification to keep the special case list_lru_dispose_all in tree.

Signed-off-by: Glauber Costa <glommer@openvz.org>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>