drivers: dma-coherent: Move spinlock in dma_alloc_from_coherent()

We don't need to hold the spinlock while zeroing the allocated memory.
In case we handle big buffers this is a severe issue as other CPUs might
be spinning half a second or longer.

Signed-off-by: Bastian Hecht <bhecht@de.adit-jv.com>
Signed-off-by: George G. Davis <george_davis@mentor.com>
Signed-off-by: Mark Craske <Mark_Craske@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>