IB/uverbs: Safely extend existing attributes

Previously, we've used UVERBS_ATTR_SPEC_F_MIN_SZ for extending existing
attributes. The behavior of this flag was the kernel accepts anything
bigger than the minimum size it specified. This is unsafe, since in
order to safely extend an attribute, we need to make sure unknown size
is zeroed. Replacing UVERBS_ATTR_SPEC_F_MIN_SZ with
UVERBS_ATTR_SPEC_F_MIN_SZ_OR_ZERO, which essentially checks that the
unknown size is zero. In addition, attributes are now decorated with
UVERBS_ATTR_TYPE and UVERBS_ATTR_STRUCT, so we can provide the minimum
and known length.

Users of this flag needs to use copy_from_or_zero functions/macros.

Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>