RDMA/rxe: Don't overwrite errno from ib_umem_get()

RDMA/rxe: Don't overwrite errno from ib_umem_get()

rxe_mr_init_user() always returns the fixed -EINVAL when ib_umem_get()
fails so it's hard for user to know which actual error happens in
ib_umem_get(). For example, ib_umem_get() will return -EOPNOTSUPP when
trying to pin pages on a DAX file.

Return actual error as mlx4/mlx5 does.

Link: https://lore.kernel.org/r/20210621071456.4259-1-ice_yangxiao@163.com
Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>