IB/uverbs: Signedness bug in UVERBS_HANDLER()

The "num_sge" variable needs to be signed for the error handling to work.
The uverbs_attr_ptr_get_array_size() returns int so this change is safe.

Fixes: ad8a4496 ("IB/uverbs: Add support to advise_mr")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>