vlog: Avoid deadlock in vlog_init__() corner case.

Anything inside vlog_init__() that tried to log a message was going to
deadlock, since it would hit pthread_once() recursively and wait for the
previous call to complete.  Unfortunately, there was a VLOG_ERR call inside
vlog_init__(), only called in the corner case where the system's clock was
wrong.

This fixes the problem by rearranging code so that the logging isn't
inside the "once-only" region.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>