mm/page_alloc: fix race condition in unaccepted memory handling

In the Linux kernel, the following vulnerability has been resolved:

mm/page_alloc: fix race condition in unaccepted memory handling

The page allocator tracks the number of zones that have unaccepted memory

using static_branch_enc/dec() and uses that static branch in hot paths to

determine if it needs to deal with unaccepted memory.

Borislav and Thomas pointed out that the tracking is racy: operations on

static_branch are not serialized against adding/removing unaccepted pages

to/from the zone.

Sanity checks inside static_branch machinery detects it:

WARNING: CPU: 0 PID: 10 at kernel/jump_label.c:276 __static_key_slow_dec_cpuslocked+0x8e/0xa0

The comment around the WARN() explains the problem:

/*

* Warn about the '-1' case though; since that means a

* decrement is concurrent with a first (0->1) increment. IOW

* people are trying to disable something that wasn't yet fully

* enabled. This suggests an ordering problem on the user side.

*/

The effect of this static_branch optimization is only visible on

microbenchmark.

Instead of adding more complexity around it, remove it altogether.