Refactor mlx5_ib_alloc_ucontext() to set its response fields in a
cleaner way.

It includes,
- Move the relevant code to a self contained function.
- Calculate the response length once and drop redundant code all around.
- Reuse previously set ucontext fields once preparing the response.

The self contained function will be used in next patch as part of
implementing the query ucontext functionality.

Link: https://lore.kernel.org/r/20200630093916.332097-5-leon@kernel.orgSigned-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>