mm, oom: prevent soft lockup on memcg oom for UP systems

When a process is oom killed as a result of memcg limits and the victim
is waiting to exit, nothing ends up actually yielding the processor back
to the victim on UP systems with preemption disabled.  Instead, the
charging process simply loops in memcg reclaim and eventually soft
lockups.

Memory cgroup out of memory: Killed process 808 (repro) total-vm:41944kB, anon-rss:35344kB, file-rss:504kB, shmem-rss:0kB, UID:0 pgtables:108kB oom_score_adj:0
watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [repro:806]
CPU: 0 PID: 806 Comm: repro Not tainted 5.6.0-rc5+ torvalds#136
RIP: 0010:shrink_lruvec+0x4e9/0xa40
...
Call Trace:
 shrink_node+0x40d/0x7d0
 do_try_to_free_pages+0x13f/0x470
 try_to_free_mem_cgroup_pages+0x16d/0x230
 try_charge+0x247/0xac0
 mem_cgroup_try_charge+0x10a/0x220
 mem_cgroup_try_charge_delay+0x1e/0x40
 handle_mm_fault+0xdf2/0x15f0
 do_user_addr_fault+0x21f/0x420
 page_fault+0x2f/0x40

Make sure that something ends up actually yielding the processor back to
the victim to allow for memory freeing.  Most appropriate place appears to
be shrink_node_memcgs() where the iteration of all decendant memcgs could
be particularly lengthy.

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: David Rientjes <rientjes@google.com>

mm/vmscan.c

@@ -2637,6 +2637,8 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 		unsigned long reclaimed;
 		unsigned long scanned;
 
+		cond_resched();
+
 		switch (mem_cgroup_protected(target_memcg, memcg)) {
 		case MEMCG_PROT_MIN:
 			/*