RDMA/uverbs: Add a check for uverbs_attr_get to uverbs_copy_to_struct_or_zero

All current callers for uverbs_copy_to_struct_or_zero() already check that
the attribute exists, but it make sense to verify the result like the
other functions do.

Link: https://lore.kernel.org/r/20191018081533.8544-1-hslester96@gmail.com


Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>