RMDA/odp: Consolidate umem_odp initialization

This is done in two different places, consolidate all the post-allocation
initialization into a single function.

Link: https://lore.kernel.org/r/20190819111710.18440-5-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>