Bump the max number of inodes for /tmp to a million too

Fixes #21626. (The bug report talks about /run, but the issue is actually with
/tmp.) People use /tmp for various things that fit in memory, e.g. unpacking
packages, and 400k is not much. Let's raise is a bit.

(cherry picked from commit cac372a8)
(cherry picked from commit d7619294)