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/[email protected]
Fixes: 7e659650cbda ("lib: introduce test_meminit module")
Signed-off-by: Alexander Potapenko <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Reported-by: kernel test robot <[email protected]>
Cc: Kees Cook <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>