reiser4: discard support: use reiser4_post_write_back_hook() for discarding and completing deferred deallocations.

This hook is now called after immediate deallocations in the wandered
logs code.

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>
reiser4: discard support: perform discard before all deallocations.

When discard is enabled, immediate deallocations are made deferred,
and thus discard procedure takes place right before any deallocation
is made. This allows to avoid costly bitmap checks at discard time.

Note that this commit itself is incomplete and needs a modified
discard_extent() function which does not check discarded extents
in the bitmap.

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