RDMA/i40iw: Address an mmap handler exploit in i40iw

i40iw_mmap manipulates the vma->vm_pgoff to differentiate a push page mmap
vs a doorbell mmap, and uses it to compute the pfn in remap_pfn_range
without any validation. This is vulnerable to an mmap exploit as described
in: https://lore.kernel.org/r/20201119093523.7588-1-zhudi21@huawei.com

The push feature is disabled in the driver currently and therefore no push
mmaps are issued from user-space. The feature does not work as expected in
the x722 product.

Remove the push module parameter and all VMA attribute manipulations for
this feature in i40iw_mmap. Update i40iw_mmap to only allow DB user
mmapings at offset = 0. Check vm_pgoff for zero and if the mmaps are bound
to a single page.

Cc: <stable@kernel.org>
Fixes: d37498417947 ("i40iw: add files for iwarp interface")
Link: https://lore.kernel.org/r/20201125005616.1800-2-shiraz.saleem@intel.com
Reported-by: Di Zhu <zhudi21@huawei.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

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