Btrfs: try to cleanup delayed refs while freeing extents

When extents are freed, it is likely that we've removed the last
delayed reference update for the extent.  This checks the delayed
ref tree when things are freed, and if no ref updates area left it
immediately processes the delayed ref.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --cc fs/btrfs/delayed-ref.c
Simple merge

diff --cc fs/btrfs/delayed-ref.h
Simple merge

diff --cc fs/btrfs/extent-tree.c
Simple merge