Fix access-outside-array of o->rwmix[]

gcc 4.9 rightfully complains about it, though in a very obtuse way:

     CC cconv.o
In file included from thread_options.h:5:0,
                  from cconv.c:3:
cconv.c: In function 'convert_thread_options_to_cpu':
os/os.h:197:16: warning: iteration 2u invokes undefined behavior
[-Waggressive-loop-optimizations]
   __le32_to_cpu(*__val);   \
                 ^
os/os.h:176:28: note: in definition of macro '__le32_to_cpu'
  #define __le32_to_cpu(x)  (x)
                             ^
cconv.c:78:17: note: in expansion of macro 'le32_to_cpu'
    o->rwmix[i] = le32_to_cpu(top->rwmix[i]);
                  ^
cconv.c:63:2: note: containing loop
   for (i = 0; i < DDIR_RWDIR_CNT; i++) {
   ^
In file included from thread_options.h:5:0,
                  from cconv.c:3:
cconv.c: In function 'convert_thread_options_to_net':
os/os.h:209:16: warning: iteration 2u invokes undefined behavior
[-Waggressive-loop-optimizations]
   __cpu_to_le32(*__val);   \
                 ^
os/os.h:179:28: note: in definition of macro '__cpu_to_le32'
  #define __cpu_to_le32(x)  (x)
                             ^
cconv.c:372:19: note: in expansion of macro 'cpu_to_le32'
    top->rwmix[i] = cpu_to_le32(o->rwmix[i]);
                    ^
cconv.c:352:2: note: containing loop
   for (i = 0; i < DDIR_RWDIR_CNT; i++) {
   ^
In file included from thread_options.h:5:0,
                  from cconv.c:3:
cconv.c: In function 'fio_test_cconv':
os/os.h:209:16: warning: iteration 2u invokes undefined behavior
[-Waggressive-loop-optimizations]
   __cpu_to_le32(*__val);   \
                 ^
os/os.h:179:28: note: in definition of macro '__cpu_to_le32'
  #define __cpu_to_le32(x)  (x)
                             ^
cconv.c:372:19: note: in expansion of macro 'cpu_to_le32'
    top->rwmix[i] = cpu_to_le32(o->rwmix[i]);
                    ^
cconv.c:352:2: note: containing loop
   for (i = 0; i < DDIR_RWDIR_CNT; i++) {
   ^
In file included from thread_options.h:5:0,
                  from cconv.c:3:
os/os.h:197:16: warning: iteration 2u invokes undefined behavior
[-Waggressive-loop-optimizations]
   __le32_to_cpu(*__val);   \
                 ^
os/os.h:176:28: note: in definition of macro '__le32_to_cpu'
  #define __le32_to_cpu(x)  (x)
                             ^
cconv.c:78:17: note: in expansion of macro 'le32_to_cpu'
    o->rwmix[i] = le32_to_cpu(top->rwmix[i]);
                  ^
cconv.c:63:2: note: containing loop
   for (i = 0; i < DDIR_RWDIR_CNT; i++) {
   ^

Fix it by extending the rwmix indexes to the full rwdir count,
even though it doesn't support trim as part of the mix (yet).

Reported-by: Bruce Cran <bruce@cran.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>