IB/uverbs: Provide implementation private memory for the uverbs_attr_bundle

This already existed as the anonymous 'ctx' structure, but this was not
really a useful form. Hoist this struct into bundle_priv and rework the
internal things to use it instead.

Move a bunch of the processing internal state into the priv and reduce the
excessive use of function arguments.

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