Fix: truncated len in lttng_event_rule_user_tracepoint_serialize()

Fix: truncated len in lttng_event_rule_user_tracepoint_serialize()

Observed issue
==============

On 64-bit big-endian platforms, the serialization/deserialization tests
of tracepoint event rules fail since the length of individual exclusions
is truncated in lttng_event_rule_user_tracepoint_serialize and appear as
"0" on the receiving end.

Cause
=====

The length of the exclusion name string is stored in a variable of type
`size_t`. However, since the protocol expects it to be expressed as a
uint32_t, the value is added to the payload by copying the first 4 bytes
of the value.

On a 32-bit system this would be fine since `sizeof(size_t) == 4`. Even
worse, it would work most of the time (assuming an exclusion name string
< 4GiB) on a little-endian 64-bit system as the least significant bits
would be copied and correctly express the length of the string.

On a big-endian 64-bit platform, the most-significant 4 bytes are copied
to the payload buffer thus making the string length appear as "0".

Solution
========

A temporary variable is used to hold the "casted" value and make it safe
to copy to the payload buffer regardless of the platform's endianness.

Known drawbacks
===============

None.

Change-Id: I64c03345fff7ffea2f8fcb84692a085da31c421b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>