rdma: Add copy-on-fork to get sys command

rdma: Add copy-on-fork to get sys command

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

If the attribute is not reported (expected on older kernels),
copy-on-fork is disabled.

Example:
$ rdma sys
netns shared copy-on-fork on

Signed-off-by: Gal Pressman <galpress@amazon.com>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>