RDMA/uverbs: Annotate uverbs_request_next_ptr() return value as a __user pointer

This patch avoids that sparse complains about a mismatch between the
returned value and the function return type.

Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Fixes: c3bea3d2

 ("RDMA/uverbs: Use the iterator for ib_uverbs_unmarshall_recv()")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>