RDMA/core: Clean ib_alloc_xrcd() and reuse it to allocate XRC domain

ib_alloc_xrcd() already does the required initialization, so move the
uverbs to call it and save code duplication, while cleaning the function
argument lists of that function.

Link: https://lore.kernel.org/r/20200706122716.647338-3-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>