RDMA/mlx5: Signedness bug in UVERBS_HANDLER()

The "num_actions" variable needs to be signed for the error handling to
work.  The maximum number of actions is less than 256 so int type is large
enough for that.

Fixes: cbfdd442 ("IB/uverbs: Add helper to get array size from ptr attribute")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>