dma-coherent: fix bitmap access races

The bitmap functions, though, do not protect the bitmap structure from
being modified concurrently. This can lead, for example, to double
allocations if dma_alloc_from_coherent() is called while another
dma_alloc_from_coherent() is already in progress.

Fix those races by protecting concurrent modifications of the allocation
bitmap. spin_lock_irqsave()/spin_unlock_irqrestore() is used as the
allocation/release functions are planned to be used in interrupt context
for streaming DMA mappings/unmappings via bounce buffers.