If the user did not provide a long enough command buffer then the missing
bytes are forced to zero. There is no reason to check the length if a zero
value is OK.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>