Btrfs: tweak congestion backoff

The btrfs io submission thread tries to back off congested devices in
favor of rotating off to another disk.

But, it tries to make sure it submits at least some IO before rotating
on (the others may be congested too), and so it has a magic number of
requests it tries to write before it hops.

This makes the magic number smaller.  Testing shows that we're spending
too much time on congested devices and leaving the other devices idle.

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