ghostpad: hold a reference to the target pad while unlinking it

https://bugzilla.gnome.org/show_bug.cgi?id=725809
ghostpad: use gst_pad_get_peer to acquire a reference to the target pad

This ensures that the lock of the internal pad is held while referencing
it's peer (= the target pad), which ensures that the peer is not
going to be unlinked/destroyed in the meantime.

https://bugzilla.gnome.org/show_bug.cgi?id=725809
fakesink, identity, fakesrc: fix debug printing if TAG_MEMORY buffer flag

The IN_CAPS flag does not exist any more.