mm/zsmalloc.c: make several functions and a struct static

mm/zsmalloc.c: make several functions and a struct static

The functions zs_page_isolate, zs_page_migrate, zs_page_putback,
lock_zspage, trylock_zspage and structure zsmalloc_aops are local to
source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
  symbol 'zs_page_isolate' was not declared. Should it be static?
  symbol 'zs_page_migrate' was not declared. Should it be static?
  symbol 'zs_page_putback' was not declared. Should it be static?
  symbol 'zsmalloc_aops' was not declared. Should it be static?
  symbol 'lock_zspage' was not declared. Should it be static?
  symbol 'trylock_zspage' was not declared. Should it be static?

[arnd@arndb.de: hide unused lock_zspage]
Link: http://lkml.kernel.org/r/20180706130924.3891230-1-arnd@arndb.de
Link: http://lkml.kernel.org/r/20180624213322.13776-1-colin.king@canonical.com
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>