Kyeongdon reported below error which is BUG_ON(!PageSwapCache(page)) in
page_swap_info.  The reason is that page_endio in rw_page unlocks the
page if read I/O is completed so we need to hold a PG_lock again to
check PageSwapCache.  Otherwise, the page can be removed from swapcache.

  Kernel BUG at c00f9040 [verbose debug info unavailable]
  Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
  Modules linked in:
  CPU: 4 PID: 13446 Comm: RenderThread Tainted: G        W 3.10.84-g9f14aec-dirty #73
  task: c3b73200 ti: dd192000 task.ti: dd192000
  PC is at page_swap_info+0x10/0x2c
  LR is at swap_slot_free_notify+0x18/0x6c
  pc : [<c00f9040>]    lr : [<c00f5560>]    psr: 400f0113
  sp : dd193d78  ip : c2deb1e4  fp : da015180
  r10: 00000000  r9 : 000200da  r8 : c120fe08
  r7 : 00000000  r6 : 00000000  r5 : c249a6c0  r4 : = c249a6c0
  r3 : 00000000  r2 : 40080009  r1 : 200f0113  r0 : = c249a6c0
  ..<snip> ..
  Call Trace:
    page_swap_info+0x10/0x2c
    swap_slot_free_notify+0x18/0x6c
    swap_readpage+0x90/0x11c
    read_swap_cache_async+0x134/0x1ac
    swapin_readahead+0x70/0xb0
    handle_pte_fault+0x320/0x6fc
    handle_mm_fault+0xc0/0xf0
    do_page_fault+0x11c/0x36c
    do_DataAbort+0x34/0x118

Fixes: 3f2b1a04 ("zram: revive swap_slot_free_notify")
Signed-off-by: Minchan Kim <minchan@kernel.org>
Tested-by: Kyeongdon Kim <kyeongdon.kim@lge.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>