libostree: Silently ignore EPERM when setting EXT2_IMMUTABLE_FL

In the case of running ostree as non-root on a regular filesystem (not
tmpfs which doesn't support immutable), we should just silently do
nothing if we encounter EPERM.  Cache the result to avoid spam in
strace.

https://bugzilla.gnome.org/show_bug.cgi?id=728006