[PATCH RFC] __futex_pivot_hash() race

[Chris Mason]

This is really just AI questioning how we synchronize mmph->batches and
mmph->hash.  It looks real to me, and the repro with printks does fire,
so hopefully I've got things right.

The patch and description were also AI, meaning this is more bug
report in patch form.  Testing was only done against the one repro.

(I may or may not have successfully switched over my git config to
mason@xxxxxxxxxx, lets see...)