IB/mlx5: Extend QP creation to get uar page index from user space

Extend QP creation to get uar page index from user space, this mode can be
used with the UAR dynamic mode APIs to allocate/destroy a UAR object.

As part of enabling this option blocked the weird/un-supported cross
channel option which uses index 0 hard-coded.

This QP flag wasn't exposed to user space as part of any formal upstream
release, the dynamic option can allow having valid UAR page index instead.

Link: https://lore.kernel.org/r/20200324060143.1569116-4-leon@kernel.org
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Reviewed-by: Michael Guralnik <michaelgur@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>