This is the promised generalization, which is supposed to work for all 
discard
offsets and all discard unit sizes without any restrictions.

Complications in comparison with the previous implementation:

In this general case we need "precise" coordinates, where every 
individual byte
can be addressed. All local variables, which represent precise 
coordinates are
denoted with "prefixes" (a_len, d_off, p_tailp, etc). Local variables, 
which represent
"non-precise" coordinates (they are usually of type reiser4_block_nr) 
are denoted
without prefixes (start, len, end, tailp, etc).

Blocks, which contain head and tail paddings are now calculated using the
function size_in_blocks(), which actually is an expression for the 
minimal number
of blocks containing the precise extent.

The next trouble is "peculiarity in 0", encountered when calculating the 
blocks of
head padding. if discard offset is different from 0, then the first 
discard unit of the
partition is partial (its other part doesn't belong to our partition, so 
we can not
discard it). We handle this peculiarity by an additional check.

In other bits everything is the same.

Possible optimization: If discard unit sizes are always powers of 2, 
then it makes
sense to replace "do_div(offset, unit_size)" with "offset & (unit_size - 
1)".

Mount options discard.offset=xxx,discard.unit=yyy are to emulate various
discard unit sizes and offsets on devices _without_ trim support (e.g. 
HDDs).
This is only for debugging purposes, don't use it for real SSD devices: 
the kernel
retrieves the discard parameters on its own.

This patch is against the patch series of Ivan Shapovalov:
http://marc.info/?l=reiserfs-devel&m=141841865432082&w=2

Current status: not well-tested.

Edward.