async_tx: remove HIGHMEM64G restriction

async_tx: remove HIGHMEM64G restriction

This restriction prevented ASYNC_TX_DMA from being enabled on platform
configurations where DMA address conversion could not be performed in
place on the stack.  Since commit 04ce9ab3 ("async_xor: permit callers
to pass in a 'dma/page scribble' region") the async_tx api now either
uses a caller provided 'scribble' buffer, or performs the conversion in
place when sizeof(dma_addr_t) <= sizeof(struct page *).

Signed-off-by: Dan Williams <dan.j.williams@intel.com>