btrfs: subpage: fix race between prepare_pages() and btrfs_releasepage()

btrfs: subpage: fix race between prepare_pages() and btrfs_releasepage()

[BUG]
When running generic/095, there is a high chance to crash with subpage
data RW support:

 assertion failed: PagePrivate(page) && page->private
 ------------[ cut here ]------------
 kernel BUG at fs/btrfs/ctree.h:3403!
 Internal error: Oops - BUG: 0 [#1] SMP
 CPU: 1 PID: 3567 Comm: fio Tainted: 5.12.0-rc7-custom+ #17
 Hardware name: Khadas VIM3 (DT)
 Call trace:
  assertfail.constprop.0+0x28/0x2c [btrfs]
  btrfs_subpage_assert+0x80/0xa0 [btrfs]
  btrfs_subpage_set_uptodate+0x34/0xec [btrfs]
  btrfs_page_clamp_set_uptodate+0x74/0xa4 [btrfs]
  btrfs_dirty_pages+0x160/0x270 [btrfs]
  btrfs_buffered_write+0x444/0x630 [btrfs]
  btrfs_direct_write+0x1cc/0x2d0 [btrfs]
  btrfs_file_write_iter+0xc0/0x160 [btrfs]
  new_sync_write+0xe8/0x180
  vfs_write+0x1b4/0x210
  ksys_pwrite64+0x7c/0xc0
  __arm64_sys_pwrite64+0x24/0x30
  el0_svc_common.constprop.0+0x70/0x140
  do_el0_svc+0x28/0x90
  el0_svc+0x2c/0x54
  el0_sync_handler+0x1a8/0x1ac
  el0_sync+0x170/0x180
 Code: f0000160 913be042 913c4000 955444bc (d4210000)
 ---[ end trace 3fdd39f4cccedd68 ]---

[CAUSE]
Although prepare_pages() calls find_or_create_page(), which returns the
page locked, but in later prepare_uptodate_page() calls, we may call
btrfs_readpage() which will unlock the page before it returns.

This leaves a window where btrfs_releasepage() can sneak in and release
the page, clearing page->private and causing above ASSERT().

[FIX]
In prepare_uptodate_page(), we should not only check page->mapping, but
also PagePrivate() to ensure we are still holding the correct page which
has proper fs context setup.

Reported-by: Ritesh Harjani <riteshh@linux.ibm.com>
Tested-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>