mm/page_alloc: Revert pahole zero-sized workaround

mm/page_alloc: Revert pahole zero-sized workaround

Commit f44d12cf8ab2 ("mm/page_alloc: convert per-cpu list protection to
local_lock") folded in a workaround patch for pahole that was unable to
deal with zero-sized percpu structures.

A superior workaround is achieved with commit 0da17db61383 ("kbuild:
skip per-CPU BTF generation for pahole v1.18-v1.21").

This patch reverts the dummy field and the pahole version check.

Fixes: f44d12cf8ab2 ("mm/page_alloc: convert per-cpu list protection to local_lock")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>