f2fs: fix atgc bug on issue in 32bits platform

There is bug on issue after atgc feature is enabled in
32bits platform as the following log:

F2FS-fs (dm-x): inconsistent rbtree, cur(3470333575168) next(3320009719808)
------------[ cut here ]------------
kernel BUG at fs/f2fs/gc.c:602!
Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
PC is at get_victim_by_default+0x13c0/0x1498
LR is at f2fs_check_rb_tree_consistence+0xc4/0xd4
....
[<c04d98b0>] (get_victim_by_default) from [<c04d4f44>] (f2fs_gc+0x220/0x6cc)
[<c04d4f44>] (f2fs_gc) from [<c04d4780>] (gc_thread_func+0x2ac/0x708)
[<c04d4780>] (gc_thread_func) from [<c015c774>] (kthread+0x1a8/0x1b4)
[<c015c774>] (kthread) from [<c01010b4>] (ret_from_fork+0x14/0x20)

the reason is the 64bits key in struct rb_entry has __packed attibute
but has not in struct victim_entry, so the wrong key value got by
struct rb_entry in f2fs_check_rb_tree_consistence, the following are
the memory layouts of struct rb_entry and struct victim_entry in 32bits
platform:

struct rb_entry {
   [0] struct rb_node rb_node;
       union {
           struct {...};
  [12]     unsigned long long key;
       };
}
SIZE: 20

struct victim_entry {
   [0] struct rb_node rb_node;
       union {
           struct {...};
  [16]     struct victim_info vi;
       };
  [32] struct list_head list;
}
SIZE: 40

This patch fix the inconsistence layout of 64bits key between
struct rb_entry and struct victim_entry.

Fixes: 093749e296e2 ("f2fs: support age threshold based garbage collection")

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>