RDMA/core: Create clean QP creations interface for uverbs

Unify create QP creation interface to make clean approach to create
XRC_TGT and regular QPs.

Link: https://lore.kernel.org/r/5cd50e7d8ad9112545a1a61dea62799a5cb3224a.1628014762.git.leonro@nvidia.com


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