Btrfs: fix up seek_hole/seek_data handling

Whoever wrote this was braindead.  Also it doesn't work right if you have
VACANCY's since we assumed you would only have that at the end of the file,
which won't be the case in the near future.  I tested this with generic/285 and
generic/286 as well as the btrfs tests that use fssum since it uses
seek_hole/seek_data to verify things are ok.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>