evdev: use a prometheus-safe label for ev_sysctl_tree

Prometheus doesn't allow spaces, let's normalize this to what we use
elsewhere for consistency. The sysctl exporter could probably do this
itself, but let's decouple that from the immediate problem: matching the
label between the exported data and in-tree is nice for greppability.

PR: 296179
Reviewed by: asomers, wulf
Differential Revision: https://reviews.freebsd.org/D57966