IB: Fix ib_dma_alloc_coherent() wrapper

The ib_dma_alloc_coherent() wrapper uses a u64* for the dma_handle
parameter, unlike dma_alloc_coherent, which uses dma_addr_t*.  This
means that we need a temporary variable to handle the case when
ib_dma_alloc_coherent() just falls through directly to
dma_alloc_coherent() on architectures where sizeof u64 != sizeof
dma_addr_t.

Signed-off-by: Roland Dreier <rolandd@cisco.com>