f2fs: factor out discard_cmd usage from general rb_tree use

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: 004b68621897 ("f2fs: use rb-tree to track pending discard commands")
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>