memcg: remove redundant message at swapon

It's pointed out that swap_cgroup's message at swapon() is nonsense.
Because

  * It can be calculated very easily if all necessary information is
    written in Kconfig.

  * It's not necessary to annoying people at every swapon().

In other view, now, memory usage per swp_entry is reduced to 2bytes from
8bytes(64bit) and I think it's reasonably small.

Reported-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -597,6 +597,8 @@ config CGROUP_MEM_RES_CTLR_SWAP
 	  is disabled by boot option, this will be automatically disabled and
 	  there will be no overhead from this. Even when you set this config=y,
 	  if boot option "noswapaccount" is set, swap will not be accounted.
+	  Now, memory usage of swap_cgroup is 2 bytes per entry. If swap page
+	  size is 4096bytes, 512k per 1Gbytes of swap.
 
 endif # CGROUPS