Btrfs: Handle data block end_io through the async work queue

Before it was done by the bio end_io routine, the work queue code is able
to scale much better with faster IO subsystems.

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

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

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

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