In prealloc_file_extent_cluster(), btrfs_check_data_free_space() uses
wrong file offset for reloc_inode, it uses cluster->start and cluster->end,
which indeed are extent's bytenr. The correct value should be
cluster->[start|end] minus block group's start bytenr.

start bytenr   cluster->start
|              |     extent      |   extent   | ...| extent |
|----------------------------------------------------------------|
|                block group reloc_inode                         |
Signed-off-by: NWang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NChris Mason <clm@fb.com>