mm: make zone_pcp_reset independent of MEMORY_HOTREMOVE

mm: make zone_pcp_reset independent of MEMORY_HOTREMOVE

340175b7 (mm/hotplug: free zone->pageset when a zone becomes empty)
introduced zone_pcp_reset and hided it inside CONFIG_MEMORY_HOTREMOVE.
Since "memory-hotplug: allocate zone's pcp before onlining pages" the
function is also called from online_pages which is defined outside
CONFIG_MEMORY_HOTREMOVE which causes a linkage error.

The function, although not used outside of MEMORY_{HOTPLUT,HOTREMOVE},
seems like universal enough so let's keep it at its current location
and only remove the HOTREMOVE guard.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Wen Congyang <wency@cn.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>