Add per-channel EDAC error metrics with DIMM labels. (#3720)

Expose channel-level correctable and uncorrectable error counters with
dimm_label from sysfs, keep existing controller and csrow metrics, and
fail fast when channel count files are present but unreadable.

Signed-off-by: Arun S <arun.srinivasan@flipkart.com>
Co-authored-by: Arun S <arun.srinivasan@flipkart.com>
Co-authored-by: Cursor <cursoragent@cursor.com>