reiser4: discard support: initial implementation using blocknr_list, without extent padding.

Now reiser4_post_write_back_hook() is used for discarding and completing
deferred deallocations. The invocation of this hook has been moved past
immediate deallocations in the wandered logs code. Also, when discard is
enabled, immediate deallocations are made deferred, and discard procedure
takes place before any deallocations are completed. This allows to avoid
costly bitmap checks at discard time.

Implementation details:

- before discarding, the delete set is sorted and extents are merged
- each extent is submitted to discard as-is (blocks in all extents
  are still marked as allocated at this point, so there can be no races)
- processing stops at first failure (this does not fail atom commit)

For now (shortcomings):

- extents are not padded to erase unit boundaries

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>