This merges a fix for a bug in our context id handling on 64-bit hash
CPUs.

The fix was written against v5.1 to ease backporting to stable
releases. Here we are merging it up to a v5.2-rc2 base, which involves
a bit of manual resolution.

It also adds a test case for the bug.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>