Btrfs: don't split dio bios if we don't have to

We have been unconditionally allocating a new bio and re-adding all pages from
our original bio to the new bio.  This is needed if our original bio is larger
than our stripe size, but if it is smaller than the stripe size then there is no
need to do this.  So check the map length and if we are under that then go ahead
and submit the original bio.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>