lib/test_meminit.c: use GFP_ATOMIC in RCU critical section

lib/test_meminit.c: use GFP_ATOMIC in RCU critical section

kmalloc() shouldn't sleep while in RCU critical section, therefore use
GFP_ATOMIC instead of GFP_KERNEL.

The bug was spotted by the 0day kernel testing robot.

Link: http://lkml.kernel.org/r/20190725121703.210874-1-glider@google.com
Fixes: 7e659650cbda ("lib: introduce test_meminit module")
Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>