reiser4: discard support: discard before performing deallocations; simplify discard_extent().

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.

discard_extent() has been simplified not to check allocation status of extents
and not to do any padding of extents (as this introduces allocation races and
needs to be done properly).

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