Fix bitmap access races

The coherent per-device memory handling functions use the in-kernel
bitmap library to account for the allocated regions.

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.