Btrfs: factor page private preparations into a helper

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

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

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

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

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