journal: return mapped size from mmap_cache_get()

If requested, return the actual mapping size to the caller in
addition to the address.

journal_file_move_to_object() often performs two successive
mmap_cache_get() calls via journal_file_move_to(); one to get the
object header, then another to get the entire object when it's
larger than the header's size.

If mmap_cache_get() returned the actual mapping's size, it's
probable that the second mmap_cache_get() could be skipped when
the established mapping already encompassed the desired size.