Fix: lttng-ust-comm.c: return number of fd rather size of array

Fix: lttng-ust-comm.c: return number of fd rather size of array

There are two conflicting comments for this function. One says it
returns the size of the received data and the other says it returns the
number of fd received.

It's more useful to receive the number of fd.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I74084b461d396c3e623fa55100e6dd7e59dbea83