Btrfs: extent map selftest: dio write vs dio read

This test case simulates the racy situation of dio write vs dio read,
and see if btrfs_get_extent() would return -EEXIST.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>