If we have

|0--hole--4095||4096--preallocate--12287|

instead of using preallocated space, a 8K direct write will just
create a new 8K extent and it'll end up with

|0--new extent--8191||8192--preallocate--12287|

It's because we find a hole em and then go to create a new 8K
extent directly without adjusting @len.
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Reviewed-by: NChris Mason <clm@fb.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>