mmap-cache: drop ret_size from mmap_cache_get()

The ret_size result is a bit of an awkward optimization that in a
sense enables bypassing the mmap-cache API, while encouraging
duplication of logic it already implements.

It's only utilized in one place; journal_file_move_to_object(),
apparently to avoid the overhead of remapping the whole object
again once its header, and thus its actual size, is known.

With mmap-cache's context cache, the overhead of simply
re-getting the object with the now known size should already be
negligible.  So it's not clear what benefit this brings, unless
avoiding some function calls that do very little in the hot
context-cache hit case is of such a priority.

There's value in having all object-sized gets pass through
mmap_cache_get(), as it provides a single entrypoint for
instrumentation in profiling/statistics gathering.  When
journal_file_move_to_object() bypasses getting the full object
size, you don't capture the full picture on the mmap-cache side
in terms of object sizes explicitly loaded from a journal file.

I'd like to see additional accounting in mmap_cache_get() in a
future commit, taking advantage of this change.