GFile: Clean up file_copy_fallback to fix SEGV with btrfs

Ok, this function was just an awful mess before.  Now the problem
domain is not trivial, and I won't claim this new code is *beautiful*,
but it should fix the bug at hand, and be somewhat less prone to
failure for the next person who tries to modify it.  There's only one
unref call for each object now.

https://bugzilla.gnome.org/show_bug.cgi?id=692408