IB/core: rename pd->local_mr to pd->__internal_mr

This has two reasons: a) to clearly mark that drivers don't have any
business using it, and b) because we're going to use it for the
(dangerous) global rkey soon, so that drivers don't create on themselves.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>