The ioctl parser framework wrongly assumed that each namespace is
populated. This could lead to NULL dereferences. Fix the parser to
always check that a given namespace indeed exists.

Fixes: fac9658c ("IB/core: Add new ioctl interface")
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>