Btrfs: Write bio checksumming outside the FS mutex

This significantly improves streaming write performance by allowing
concurrency in the data checksumming.

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

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

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

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