RDMA/umem: Move to allocate SG table from pages

Remove the implementation of ib_umem_add_sg_table and instead
call to __sg_alloc_table_from_pages which already has the logic to
merge contiguous pages.

Besides that it removes duplicated functionality, it reduces the
memory consumption of the SG table significantly. Prior to this
patch, the SG table was allocated in advance regardless consideration
of contiguous pages.

In huge pages system of 2MB page size, without this change, the SG table
would contain x512 SG entries.
E.g. for 100GB memory registration:

	 Number of entries	Size
Before 	      26214400          600.0MB
After            51200		  1.2MB

Link: https://lore.kernel.org/r/20201004154340.1080481-5-leon@kernel.org


Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>