Revert "writeback: ensure that WB_SYNC_NONE writeback with sb pinned is sync"

This reverts commit 7c8a3554c683f512dbcee26faedb42e4c05f12fa.

We are investigating a hang associated with the WB_SYNC_NONE changes,
so revert them for now.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
1 file changed