RDMA/core: Check for error instead of success in alloc MR function

The common kernel pattern is to check for error, not success.  Flip the if
statement accordingly and keep the main flow unindented.

Link: https://lore.kernel.org/r/20200706120343.10816-2-galpress@amazon.com


Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>