mm/vmstat: protect per cpu variables with preempt disable on RT

Disable preemption on -RT for the vmstat code.  On vanila the code runs in
IRQ-off regions while on -RT it may not when stats are updated under a
local_lock.  "preempt_disable" ensures that the same resources is not
updated in parallel due to preemption.

This patch differs from the preempt-rt version where __count_vm_event and
__count_vm_events are also protected.  The counters are explicitly
"allowed to be to be racy" so there is no need to protect them from
preemption.  Only the accurate page stats that are updated by a
read-modify-write need protection.  This patch also differs in that a
preempt_[en|dis]able_rt helper is not used.  As vmstat is the only user of
the helper, it was suggested that it be open-coded in vmstat.c instead of
risking the helper being used in unnecessary contexts.

Link: https://lkml.kernel.org/r/20210805160019.1137-2-mgorman@techsingularity.net
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>