Btrfs: Optimize csum insertion to create larger items when possible

This reduces the number of calls to btrfs_extend_item and greatly lowers
the cpu usage while writing large files.

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