IB/ipoib: Scatter-Gather support in connected mode

IB/ipoib: Scatter-Gather support in connected mode

By default, IPoIB-CM driver uses 64k MTU. Larger MTU gives better
performance.
This MTU plus overhead puts the memory allocation for IP based packets at
32 4k pages (order 5), which have to be contiguous.
When the system memory under pressure, it was observed that allocating 128k
contiguous physical memory is difficult and causes serious errors (such as
system becomes unusable).

This enhancement resolve the issue by removing the physically contiguous
memory requirement using Scatter/Gather feature that exists in Linux stack.

With this fix Scatter-Gather will be supported also in connected mode.

This change reverts some of the change made in commit 20a4baf9c5a8
("IPoIB/cm: Reduce connected mode TX object size").

The ability to use SG in IPoIB CM is possible because the coupling
between NETIF_F_SG and NETIF_F_CSUM was removed in commit
7ce938c3de6b ("net: Kill link between CSUM and SG features.")

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Acked-by: Christian Marie <christian@ponies.io>
Signed-off-by: Doug Ledford <dledford@redhat.com>