igb_uio: fix build with kernel < 3.2

igb_uio: fix build with kernel < 3.2

Recently added "dma_zalloc_coherent()" call is causing build error
for Linux kernels < 3.2.

compile error:
lib/librte_eal/linuxapp/igb_uio/igb_uio.c:
  In function ‘igbuio_pci_probe’:
lib/librte_eal/linuxapp/igb_uio/igb_uio.c:434:2:
error: implicit declaration of function ‘dma_zalloc_coherent’
  [-Werror=implicit-function-declaration]
  map_addr = dma_zalloc_coherent(&dev->dev, 1024,
  ^

dma_zalloc_coherent() introduced with Linux kernel 3.2, with commit
Linux: 842fa69f3e0c ("include/linux/dma-mapping.h: add dma_zalloc_coherent()")
Since it does not exist for older kernels, causing a build error.

Switched to dma_alloc_coherent() API to prevent build error.

Fixes: d287e4d41be0 ("igb_uio: map dummy DMA forcing IOMMU domain attachment")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>