FROMGIT: f2fs: factor out discard_cmd usage from general rb_tree use

This is a second part to remove the mixed use of rb_tree in discard_cmd from
extent_cache.

This should also fix arm32 memory alignment issue caused by shared rb_entry.

[struct discard_cmd]               [struct rb_entry]
[0] struct rb_node rb_node;        [0] struct rb_node rb_node;
  union {                              union {
    struct {                             struct {
[16]  block_t lstart;              [12]    unsigned int ofs;
      block_t len;                         unsigned int len;
                                         };
                                         unsigned long long key;
                                       } __packed;

Cc: <stable@vger.kernel.org>
Fixes: 004b6862 ("f2fs: use rb-tree to track pending discard commands")
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 7e9775a516ff6c1e73ee2b42ec563cafee38f42f
https: //git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)
Change-Id: I61ee2b6bcc283406ca40f0ed7c3146de1e813d61