Btrfs: fix send to deal with sparse files properly

Send was just sending everything it found, even if the extent was a hole.  This
is unpleasant for users, so just skip holes when we are sending.  This will also
skip sending prealloc extents since the send spec doesn't have a prealloc
command.  Eventually we will add a prealloc command and rev the send version so
we can send down the prealloc info.  Thanks,

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