network: update comment and log message

After 4b30f2e1, reading stable_secret
sysctl property fails with -ENOMEM, instead of -EIO.
This is due to read_full_virtual_file() uses read() as the backend while
read_one_line_file() uses fgetc(). And each functions return different
error on fails.

Anyway, the failure is harmless here. So, the log message and comment is
updated.

Closes one of the issues in #19410.

(cherry picked from commit 0e275271)
(cherry picked from commit 2417e9b5)
(cherry picked from commit 0d80d62a)