f2fs: remove entire rb_entry sharing

This is a last part to remove the memory sharing for rb_tree in extent_cache.

This should also fix arm32 memory alignment issue.

[struct extent_node]               [struct rb_entry]
[0] struct rb_node rb_node;        [0] struct rb_node rb_node;
  union {                              union {
    struct {                             struct {
[16]  unsigned int fofs;           [12]    unsigned int ofs;
      unsigned int len;                    unsigned int len;
                                         };
                                         unsigned long long key;
                                       } __packed;

Cc: <stable@vger.kernel.org>
Fixes: 13054c54 ("f2fs: introduce infra macro and data structure of rb-tree extent cache")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>