Btrfs: dummy extent buffers for tree mod log

The tree modification log needs two ways to create dummy extent buffers,
once by allocating a fresh one (to rebuild an old root) and once by
cloning an existing one (to make private rewind modifications) to it.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

diff --cc fs/btrfs/extent_io.c
Simple merge

diff --cc fs/btrfs/extent_io.h
Simple merge