journalctl: fix output when --lines is used with --grep

journalctl: fix output when --lines is used with --grep

Previously, we skip the entries before arg_lines
unconditionally, which doesn't behave correctly
when used with --grep. After this commit, when
a pattern is specified, we don't skip the entries
early, but rely on the count of the lines shown
to tell us when to stop. To achieve that we would
have to search backwards instead.

Fixes #25147