Btrfs: Add some simple throttling to wait for data=ordered and snapshot deletion

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

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

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/disk-io.h
Simple merge

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

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

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