MINOR: sink: inform the user when logs will be implicitly truncated

MINOR: sink: inform the user when logs will be implicitly truncated

Consider the following example:

 |log ring@test-ring len 2000 local0
 |
 |ring test-ring
 |  maxlen 1000

This would result in emitted logs being silently truncated to 1000 because
test-ring maxlen is smaller than the log directive maxlen.

In this patch we're adding an extra check in post_sink_resolve() to detect
this kind of confusing setups and warn the user about the implicit
truncation when DIAG mode is on.

This commit depends on:
 - "MINOR: sink: simplify post_sink_resolve function"