Btrfs: make sure not to return overlapping extents to fiemap

The btrfs fiemap code was incorrectly returning duplicate or overlapping
extents in some cases.  cp was blindly trusting this result and we would
end up with a destination file that was bigger than the original because
some bytes were copied twice.

The fix here adjusts our offsets to make sure we're always moving
forward in the fiemap results.

Signed-off-by: Chris Mason <chris.mason@oracle.com>