vmstat: make vmstat_update deferrable

Commit 0eb77e98 ("vmstat: make vmstat_updater deferrable again and
shut down on idle") made vmstat_shepherd deferrable.  vmstat_update
itself is still useing standard timer which might interrupt idle task.
This is possible because "mm, vmstat: make quiet_vmstat lighter" removed
cancel_delayed_work from the quiet_vmstat.

Change vmstat_work to use DEFERRABLE_WORK to prevent from pointless
wakeups from the idle context.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>