swiotlb: do not panic on mapping failures

All properly written drivers now have error handling in the
dma_map_single / dma_map_page callers.  As swiotlb_tbl_map_single already
prints a useful warning when running out of swiotlb pool space we can
also remove swiotlb_full entirely as it serves no purpose now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>