memcg swap: use mem_cgroup_uncharge_swap fix

Although mem_cgroup_uncharge_swap has an empty placeholder for
!CONFIG_CGROUP_MEM_RES_CTLR_SWAP the definition is placed in the
CONFIG_SWAP ifdef block so we are missing the same definition for
!CONFIG_SWAP which implies !CONFIG_CGROUP_MEM_RES_CTLR_SWAP.

This has not been an issue before, because mem_cgroup_uncharge_swap was
not called from !CONFIG_SWAP context.  But Hugh Dickins has a cleanup
patch to call __mem_cgroup_commit_charge_swapin which is defined also
for !CONFIG_SWAP.

Let's move both the empty definition and declaration outside of the
CONFIG_SWAP block to avoid the following compilation error:

  mm/memcontrol.c: In function '__mem_cgroup_commit_charge_swapin':
  mm/memcontrol.c:2837: error: implicit declaration of function 'mem_cgroup_uncharge_swap'

if CONFIG_SWAP is disabled.

Reported-by: David Rientjes <rientjes@google.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -305,6 +305,13 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
 	return vm_swappiness;
 }
 #endif
+#ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
+extern void mem_cgroup_uncharge_swap(swp_entry_t ent);
+#else
+static inline void mem_cgroup_uncharge_swap(swp_entry_t ent)
+{
+}
+#endif
 #ifdef CONFIG_SWAP
 /* linux/mm/page_io.c */
 extern int swap_readpage(struct page *);
@@ -375,13 +382,6 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 {
 }
 #endif
-#ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
-extern void mem_cgroup_uncharge_swap(swp_entry_t ent);
-#else
-static inline void mem_cgroup_uncharge_swap(swp_entry_t ent)
-{
-}
-#endif
 
 #else /* CONFIG_SWAP */