reiser4: discard: allocate extent paddings.

Subsequent deallocation of newly allocated padding blocks is ensured by
writing the actual discarded extents back to the discard set.

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>
reiser4: discard: fix gluing extents in discard_sorted_merged_extents().

Extent length (@alen) should be updated together with @end, i. e. unconditionally,
and not only if we need to regenerate the tail padding.

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