RDMA/uverbs: Use uverbs_attr_bundle to pass udata for write

Now that we have metadata describing the command format the core code can
directly compute the udata pointers and all the really ugly
ib_uverbs_init_udata() calls can be removed from the handlers.

This means all the write() handlers are no longer sensitive to the layout
of the command buffer.

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