Btrfs: Make an unplug function that doesn't unplug every spindle

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

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

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

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

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