RDMA/mlx5: Split sig_err MR data into its own xarray

The locking model for signature is completely different than ODP, do not
share the same xarray that relies on SRCU locking to support ODP.

Simply store the active mlx5_core_sig_ctx's in an xarray when signature
MRs are created and rely on trivial xarray locking to serialize
everything.

The overhead of storing only a handful of SIG related MRs is going to be
much less than an xarray full of every mkey.

Link: https://lore.kernel.org/r/20191009160934.3143-3-jgg@ziepe.ca


Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>