RDMA/uverbs: Add a __user annotation to a pointer

This patch avoids that sparse and smatch report the following:

  warning: cast removes address space of expression

Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Fixes: 3a6532c9

 ("RDMA/uverbs: Use uverbs_attr_bundle to pass udata for write")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>