drivers/block/zram/zram_drv.c: make zram_page_end_io() static

zram_page_end_io() is local to the source and does not need to be in
global scope, so make it static.

Cleans up sparse warning:

  symbol 'zram_page_end_io' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20171016173336.20320-1-colin.king@canonical.com
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -428,7 +428,7 @@ static void put_entry_bdev(struct zram *zram, unsigned long entry)
 	WARN_ON_ONCE(!was_set);
 }
 
-void zram_page_end_io(struct bio *bio)
+static void zram_page_end_io(struct bio *bio)
 {
 	struct page *page = bio->bi_io_vec[0].bv_page;