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: NShachar Raindel <raindel@mellanox.com>
Signed-off-by: NHaggai Eran <haggaie@mellanox.com>
Signed-off-by: NRoland Dreier <roland@purestorage.com>