mmc: block: Check return value of blk_get_request()

blk_get_request() can fail, always check the return value.

Fixes: 0493f6fe ("mmc: block: Move boot partition locking into a driver op")
Fixes: 3ecd8cf2 ("mmc: block: move multi-ioctl() to use block layer")
Fixes: 614f0388 ("mmc: block: move single ioctl() commands to block requests")
Fixes: 627c3ccf ("mmc: debugfs: Move block debugfs into block module")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: stable@vger.kernel.org # 4.13+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>