sd-journal: fix sd_journal_enumerate_unique skipping values

sd-journal: fix sd_journal_enumerate_unique skipping values

sd_journal_enumerate_unique will lock its mmap window to prevent it
from being released by calling mmap_cache_get with keep_always=true.
This call may return windows that are wider, but compatible with the
parameters provided to it.

This can result in a mismatch where the window to be released cannot
properly be selected, because we have more than one window matching the
parameters of mmap_cache_release. Therefore, introduce a release_cookie
to be used when releasing the window.

https://bugs.freedesktop.org/show_bug.cgi?id=79380