Btrfs: do not use async submit for small DIO io's

When looking at our DIO performance Chris said that for small IO's doing the
async submit stuff tends to be more overhead than it's worth.  With this on top
of my other fixes I get about a 17-20% speedup doing a sequential dd with 4k
IO's.  Basically if we don't have to split the bio for the map length it's small
enough to be directly submitted, otherwise go back to the async submit.  Thanks,

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