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

UPSTREAM: 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>
(cherry picked from commit 384bc41fc064bd8b12b7081aa3e81d26f3407045)
Signed-off-by: Peter Kalauskas <peskal@google.com>
Bug: 112488418
Change-Id: Ie0f250e580bc1dd16e963b5dbe5bdc429fb4cd65