sysroot: Write symlinks before calling fsync(), then rename after

sysroot: Write symlinks before calling fsync(), then rename after

There might be a race here in that we create new symlink files *after*
calling `syncfs`, and they are not guaranteed to end up on disk.

Rework the code so that we create symlinks before, and then only
rename them after (and `fsync()` the directory for good measure).

Additional-fixes-by: Giuseppe Scrivano <gscrivan@redhat.com>
Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>
This still needs verification that we're fixing a real bug; but I'm
fairly confident this won't make the fsync situation worse.

https://bugzilla.gnome.org/show_bug.cgi?id=755595