writeback: ensure that WB_SYNC_NONE writeback with sb pinned is sync

Even if the writeout itself isn't a data integrity operation, we need
to ensure that the caller doesn't drop the sb umount sem before we
have actually done the writeback.

This is a fixup for commit e913fc82

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Signed-off-by: Jens Axboe <jens.axboe@oracle.com>