log: use __log_buf() if we know buf != NULL

log: use __log_buf() if we know buf != NULL

This apparently causes complaints on cygwin:

    CC stat.o
In file included from os/os-windows.h:18:0,
                 from os/os.h:54,
                 from thread_options.h:5,
                 from fio.h:18,
                 from init.c:19:
init.c: In function ‘add_job’:
os/../log.h:24:12: error: the comparison will always evaluate as ‘true’ for the address of ‘out’ will never be NULL [-Werror=address]
  if ((buf) != NULL)    \
            ^
os/../log.h:24:12: note: in definition of macro ‘log_buf’
  if ((buf) != NULL)    \

which is utterly stupid, but hey...

Signed-off-by: Jens Axboe <axboe@kernel.dk>