mm: memcontrol: fix false-positive VM_BUG_ON() on -rt

mm: memcontrol: fix false-positive VM_BUG_ON() on -rt

On -rt, the VM_BUG_ON(!irqs_disabled()) triggers inside the memcg
swapout path because the spin_lock_irq(&mapping->tree_lock) in the
caller doesn't actually disable the hardware interrupts - which is fine,
because on -rt the tophalves run in process context and so we are still
safe from preemption while updating the statistics.

Remove the VM_BUG_ON() but keep the comment of what we rely on.

Signed-off-by: Johannes Weiner <[email protected]>
Reported-by: Clark Williams <[email protected]>
Cc: Fernando Lopez-Lezcano <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>