RDMA/qib: don't pass bogus GFP_ flags to dma_alloc_coherent

dma_alloc_coherent is an opaque allocator that only uses the GFP_ flags
for allocation context control.  Don't pass GFP_USER which doesn't make
sense for a kernel DMA allocation or __GFP_COMP which makes no sense
for an allocation that can't in any way be converted to a page pointer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jason Gunthorpe <jgg@nvidia.com>