RDMA/uverbs: Use uverbs_attr_bundle to pass udata for write_ex

The core code needs to compute the udata so we may as well pass it in the
uverbs_attr_bundle instead of on the stack. This converts the simple case
of write_ex() which already has a core calculation.

Also change the write() path to use the attrs for ib_uverbs_init_udata()
instead of on the stack. This lets the write to write_ex compatibility
path continue to follow the lead of the _ex path.

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