iommu: dma-mapping: alloc bitmap while fragmented

The bitmap size can be on the order of several pages.
In a fragmented system it may not be possible to allocate the
memory as contiguous.

Handle fragmentation by falling back to vzalloc.

Change-Id: Ie93c92705eed1bbe966d5f121b3393d8909982a6
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>