full-trace.sh: fix a nasty bug in uprobes_on()

For some weird reason, the following pipeline:

cat /tmp/uprobes | sudo tee -a /sys/kernel/debug/tracing/uprobe_events

fails halfway with an "Invalid argument error". The following
patch modifies uprobes_on() in order to write each probe into
/sys/kernel/debug/tracing/uprobe_events.