RDMA/uverbs: Optimize clearing of extra bytes in response

Clear extra bytes in response in batch manner instead
of doing it per-byte.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>