kretprobes: rename "return" to "exit"

kretprobes: rename "return" to "exit"

The kretprobes instrumentation emits <func>_entry and <func>_return.
However, pairing "entry" and "return" is not semantically right. The
term "entry" should be paired with "exit" (similarly to system call
intrumentation), since "return" should be paired with "call".

If we look at what is actually instrumented by kretprobes, we indeed
instrument the entry and exit points of a function.

Therefore, change the "<func>_return" event for "<func>_exit".

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ia541253cbb65a5afc8aaed2a6cfa77d9b296c96c