mmc_test: fix large memory allocation

- Fix mmc_test_alloc_mem.

- Use nr_free_buffer_pages() instead of sysinfo.totalram to determine
  total lowmem pages.

- Change variables containing memory sizes to unsigned long.

- Limit maximum test area size to 128MiB because that is the maximum MMC
  high capacity erase size (the maxmium SD allocation unit size is just
  4MiB)

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>