RDMA/nldev: Add copy-on-fork attribute to get sys command

The new attribute indicates that the kernel copies DMA pages on fork,
hence libibverbs' fork support through madvise and MADV_DONTFORK is not
needed.

The introduced attribute is always reported as supported since the kernel
has the patch that added the copy-on-fork behavior. This allows the
userspace library to identify older vs newer kernel versions.  Extra care
should be taken when backporting this patch as it relies on the fact that
the copy-on-fork patch is merged, hence no check for support is added.

Don't backport this patch unless you also have the following series:
commit 70e806e4 ("mm: Do early cow for pinned pages during fork() for
ptes") and commit 4eae4efa ("hugetlb: do early cow when page pinned on
src mm").

Fixes: 70e806e4 ("mm: Do early cow for pinned pages during fork() for ptes")
Fixes: 4eae4efa

 ("hugetlb: do early cow when page pinned on src mm")
Link: https://lore.kernel.org/r/20210418121025.66849-1-galpress@amazon.com
Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>