fix: add missing check for __must_check 'lttng_file_ref_put()' (v6.13)

fix: add missing check for __must_check 'lttng_file_ref_put()' (v6.13)

Add a missing return code check to a call to 'lttng_file_ref_put()'
which is marked as 'must_check', otherwise it results in a build
failure when using -Werror.

Change-Id: Ib50ec669ffc0fe87a367b25b788518d148f7a85e
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>