cxgbe/iw_cxgbe: Initialize the vma correctly in c4iw_mmap.

This fixes userspace RDMA applications that would fail due to mmap
failure. The driver's mmap routine would succeed but later the
linux_compat.c mmap routine would fail because vma->vm_private_data
wasn't set properly.

This is catch-up with b633e08c705fe43180567eae26923d6f6f98c8d9.

Reported by: Veeresh @ Chelsio
Sponsored by: Chelsio Communications