journal-file: don't use pread() when determining where to append, use mmap as before

journal-file: don't use pread() when determining where to append, use mmap as before

This partially undoes the effect of
ab6e257b3e4e5b95f3750ed019bed6e89989e41b.

Originally, we always used the mmap logic to determine the current end
of the file. ab6e257b3e4e5b95f3750ed019bed6e89989e41b changed this so
that we always used pread().

With this change we'll use pread() from the synchronization thread and
mmap otherwise.