dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING for remapped allocations

We need to return a dma_addr_t even if we don't have a kernel mapping.
Do so by consolidating the phys_to_dma call in a single place and jump
to it from all the branches that return successfully.

Fixes: bfd56cd6

 ("dma-mapping: support highmem in the generic remap allocator")
Reported-by: Liviu Dudau <liviu@dudau.co.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Liviu Dudau <liviu@dudau.co.uk>