Revert "mm: handle lru_add_drain_all for UP properly"

This reverts commit e6e9d6e2.

It was a 4.19.31 backport of commit 6ea183d6 ("mm: handle
lru_add_drain_all for UP properly").  In summary, what that did
was to fix a possible harmless WARN_ON on non-SMP, introduced at
commit 4d43d395 ("workqueue: Try to catch flush_work() without
INIT_WORK().") by adding non-SMP variants of lru functions.

The combination of that, with the -rt commit 473f14a9 ("mm:
perform lru_add_drain_all() remotely") at the merge of the two
results in the following build failure:

  mm/swap.c:736:2: error: #endif without #if

since the -rt change wants RT specific lru and the stable backport
wants non-SMP specific lru, and a chunk of the backport with
an #ifdef CONFIG_SMP is missing.

However, before we add a four way cluster of ifdeffery to handle all
cases, we note 4d43d395 was added to the v5.1 release, and it
was not (currently) backported to any 4.19.x stable release - so it is
unclear to me why this commit was ever backported to 4.19.31 at all.

Further, we note this change was to mm/swap.c -- and by definition,
any preempt-rt deployment that uses swap for anything other than a
failure contingency mitigation is broken by design.

Given all that, I decided that the best path forward was to revert
the two of the three chunks of the backport that remain in the -rt
branch, and return us to the pre-4.19.31 merge behaviour for -rt.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>