reiser4: discard: fix gluing extents (extent length update) 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>
reiser4: discard: fix gluing extents (tail padding update) in discard_sorted_merged_extents().

Typical off-by-one.

old end = 3
old tailp =  1

new end = 4
new tailp = 0
need update = (old end + old tailp < new end) = (4 < 4) = 0

Signed-off-by: Ivan Shapovalov <intelfx100@gmail.com>
reiser4: blocknrlist: add operations blocknr_list_del() and blocknr_list_update_extent().

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