RDMA/uverbs: Silence shiftTooManyBitsSigned warning

Fix reported by kbuild warning.

   drivers/infiniband/core/uverbs_cmd.c:1897:47: warning: Shifting signed 32-bit value by 31 bits is undefined behaviour [shiftTooManyBitsSigned]
    BUILD_BUG_ON(IB_USER_LAST_QP_ATTR_MASK == (1 << 31));
                                                 ^
Link: https://lore.kernel.org/r/20200720175627.1273096-3-leon@kernel.org


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>