IB/mlx5: Refactor UMR to have its own context struct

Instead of having the UMR context part of each memory region, allocate
a struct on the stack.  This allows queuing multiple UMRs that access
the same memory region.

Signed-off-by: Shachar Raindel <raindel@mellanox.com>
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>