RDMA/rxe: Fix error type of mmap_offset

The type of mmap_offset should be u64 instead of int to match the type of
mminfo.offset. If otherwise, after we create several thousands of CQs, it
will run into overflow issues.

Link: https://lore.kernel.org/r/20191227113613.5020-1-kejiewei.cn@gmail.com


Signed-off-by: Jiewei Ke <kejiewei.cn@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>