RDMA: Use the sg_table directly and remove the opencoded version from umem

This allows using the normal sg_table APIs and makes all the code
cleaner. Remove sgt, nents and nmapd from ib_umem.

Link: https://lore.kernel.org/r/20210824142531.3877007-4-maorg@nvidia.com


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