dm-verity-fec: rename "RS block" to "RS codeword"

dm-verity-fec: rename "RS block" to "RS codeword"

The literature refers to the unit of a Reed-Solomon (RS) code as either
a "block" or a "codeword".

dm-verity's source code uses "RS block".  Unfortunately, that's really
confusing because "block" already means something else in dm-verity.
Especially problematic is the fact that dm-verity sometimes uses "RS
block" to mean an RS codeword and sometimes to mean some dm-verity block
that's related to the RS decoding process, for example one of the blocks
that shares its RS codewords with the target block.

Let's use "RS codeword" instead, or "RS message" when referring to just
the message part of the codeword.  Update some comments, function names,
macro names, and variable names accordingly.  No functional change.

There are still some remaining comments where "RS block" refers to a
dm-verity block.  Later commits will handle these cases.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>