Btrfs: add inodes before dropping the extent lock in find_all_leafs

We must build up the inode list with the extent lock held after following
indirect refs.

This also requires an extension to ulists, which allows to modify the stored
aux value in case a key already exists in the list.

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

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

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

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