mmc: test: reduce stack usage in mmc_test_nonblock_transfer

The new lockdep annotations for completions cause a warning in the
mmc test module, in a function that now has four 150 byte structures
on the stack:

drivers/mmc/core/mmc_test.c: In function 'mmc_test_nonblock_transfer.constprop':
drivers/mmc/core/mmc_test.c:892:1: error: the frame size of 1360 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

The mmc_test_ongoing_transfer function evidently had a similar problem,
and worked around it by using dynamic allocation.

This generalizes the approach used by mmc_test_ongoing_transfer() and
applies it to mmc_test_nonblock_transfer() as well.

Fixes: cd8084f9 ("locking/lockdep: Apply crossrelease to completions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>