Fix: read subbuffer mmap/splice signedness issue

Signed and unsigned values were compared thus making the code path where
ret = -1 being interpreted as ret > len == true thus not cleaning up the
relayd.

This patch simplifies the read subbuffer mmap operation since
lttng_write() now provides a guarantee that the return data is either
equal to the count passed or less which the later means the endpoint has
stop working.

Signed-off-by: David Goulet <dgoulet@efficios.com>