IB/mlx4: Use pgprot_writecombine() for BlueFlame pages

The PAT work on x86 has finally made pgprot_writecombine() a usable API
for modular drivers.  As the comment indicates, this is exactly what we
want to use in mlx4_ib to map BlueFlame pages up to userspace, since
using WC for these pages improves small message latency significantly.

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