Btrfs: Use a chunk of the key flags to record the item type.

Add (untested and simple) directory item code
Fix comp_keys to use the new key ordering
Add btrfs_insert_empty_item

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

diff --cc fs/btrfs/Makefile
Simple merge

diff --cc fs/btrfs/TODO
Simple merge

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

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

diff --cc fs/btrfs/dir-item.c
Simple merge

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

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

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

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

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

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

diff --cc fs/btrfs/quick-test.c
Simple merge

diff --cc fs/btrfs/random-test.c
Simple merge

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