Btrfs: snapshot-aware defrag

This comes from one of btrfs's project ideas,
As we defragment files, we break any sharing from other snapshots.
The balancing code will preserve the sharing, and defrag needs to grow this
as well.

Now we're able to fill the blank with this patch, in which we make full use of
backref walking stuff.

Here is the basic idea,
o  set the writeback ranges started by defragment with flag EXTENT_DEFRAG
o  at endio, after we finish updating fs tree, we use backref walking to find
   all parents of the ranges and re-link them with the new COWed file layout by
   adding corresponding backrefs.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
mm/slab: add a leak decoder callback

This adds a leak decoder callback so that slab destruction
can use to generate debugging output for the allocated objects.

Callers like btrfs are using their own leak tracking which will
manage allocated objects in a list(or something else), this does
indeed the same thing as what slab does.  So adding a callback
for leak tracking can avoid this as well as runtime overhead.

(The idea is from Zach Brown <zab@zabbo.net>.)

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>