IB/uverbs: Allow an empty namespace in ioctl() framework

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>