MFV r268851:

When a sync task is waiting for a txg to complete, we should hurry it along
by increasing the number of outstanding async writes (i.e. make
vdev_queue_max_async_writes() return a larger number).

Illumos issue:
    4753 increase number of outstanding async writes when sync task is waiting

MFC after:	2 weeks

(cherry picked from commit 89f8d00f2ce7e92face7fbb2cd1921605108aee0)
(cherry picked from commit 5ab67a20b9e00a5c57327bd2a9e49951dfde2ff8)