RDMA/rxe: Fix memleak in rxe_mem_init_user

When page_address() fails, umem should be freed just like when
rxe_mem_alloc() fails.

Fixes: 8700e3e7 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20200819075632.22285-1-dinghao.liu@zju.edu.cn


Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>