RDMA/uverbs: Fix RCU annotation for radix slot deference

The uapi radix tree is a write-once data structure protected by kref.
Once we get to the ioctl() fop it is not possible for anything else
to be writing to it, so the access should use rcu_dereference_protected.

Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>