memlock: avoid type confusion in format string

memlock: avoid type confusion in format string

Windows GCC has the followng grumble:

t/memlock.c: In function 'worker':
t/memlock.c:25:26: error: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'size_t' {aka 'long long unsigned int'} [-Werror=format=]
   25 |    printf("loop%d: did %lu MiB\n", i+1, size/(1024UL*1024UL));
      |                        ~~^              ~~~~~~~~~~~~~~~~~~~~
      |                          |                  |
      |                          long unsigned int  size_t {aka long long unsigned int}
      |                        %llu

Fix it up by just using the original variable that was already in
megabytes.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>