logd: notify kernel log readers of new messages

LogBuffer::log() returns either a negative number on error or a
positive number indicating the length of the message written.
Therefore, the check to notify kernel log readers of a new message
should be that this function's return value is > 0.

Bug: 78209416
Test: `adb logcat -b kernel` updates when new log messages are present
Change-Id: Icc18c0c22e62340994e5c26aedb72282d61c1541