Btrfs: add leak debug for extent map

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Btrfs: fix memory leak on extent map after fsync

During fsync, we put the changed parts(i.e. extent map) into the log tree,
and we ship these parts from a list of modified_extents to a local list
to process, of course, we must increment the refs of the extent maps to
avoid it from getting evicted from cache.

The problem is
we don't hold the tree writer lock all the time of iterating the local list,
and it is possible that other threads hack in and delete the extent map from
the local list silently.  So we'll end up with memory leak here.

I hit this when testing xfstest 274 with mount options 'autodefrag,compress=zlib'.

With this fix, the memory leak has gone away.

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