IB/uverbs: Add a macro to define a type with no kernel known size

Sometimes the uverbs uAPI  doesn't really care about the structure it gets
from user-space. All it wants to do is to allocate enough space and send
it to the hardware/provider driver. Adding a UVERBS_ATTR_MIN_SIZE that
could be used for this scenarios. We use USHRT_MAX as the kernel known
size to bypass any zero validations.

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