RDMA/ucma: Mask QPN to be 24 bits according to IBTA

IBTA declares QPN as 24bits, mask input to ensure that kernel
doesn't get higher bits and ensure by adding WANR_ONCE() that
other CM users do the same.

Link: https://lore.kernel.org/r/20200212072635.682689-2-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>