RDMA/uverbs: Don't overwrite NULL pointer with ZERO_SIZE_PTR

RDMA/uverbs: Don't overwrite NULL pointer with ZERO_SIZE_PTR

[ Upstream commit a5cc9831af05e658543593abaee45a29d061bac4 ]

Number of specs is provided by user and in valid case can be equal to zero.
Such argument causes to call to kcalloc() with zero-length request and in
return the ZERO_SIZE_PTR is assigned. This pointer is different from NULL
and makes various if (..) checks to success.

Fixes: b6ba4a9aa59f ("IB/uverbs: Add support for flow counters")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>