RDMA/core: Add signature attrs element for ib_mr structure

This element will describe the needed characteristics for the signature
operation per signature enabled memory region (type IB_MR_TYPE_INTEGRITY).
Also add meta_length attribute to ib_sig_attrs structure for saving the
mapped metadata length (needed for the new API implementation).

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>