IB/uverbs: Fix validating mandatory attributes

Previously, if a method contained mandatory attributes in a namespace
that wasn't given by the user, these attributes weren't validated.
Fixing this by iterating over all specification namespaces.

Fixes: fac9658c ("IB/core: Add new ioctl interface")
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>