liblttng-ust-comm: move `_unlock_fd_tracker()` after `close()` on error paths

liblttng-ust-comm: move `_unlock_fd_tracker()` after `close()` on error paths

Right now, receiving an error from `lttng_ust_add_fd_to_tracker()` means that
the fd was _not_ added to the fd set. So the `lttng_ust_safe_close_fd()`
(overriding `close()`) call will indeed close the fd as expected. So, it
doesn't matter if the `close()` is before or after the `_unlock_`.

Even considering that, I believe that it's clearer and more common to
have all related operations within the `_lock_` and  `_unlock_`
functions. Also, `lttng_ust_add_fd_to_tracker()` might be modified in
the future and fail for some other reason.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Id29a6ab004cfd5ca601615e1a70c74cf754b12e2