Btrfs: update hole handling v2

This patch splits the hole insertion code out of btrfs_setattr
into btrfs_cont_expand and updates btrfs_get_extent to properly
handle the case that file extent items are not continuous.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>

diff --cc fs/btrfs/ctree.h
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