block: Eliminate the S_1KiB, S_2KiB, ... macros

block: Eliminate the S_1KiB, S_2KiB, ... macros

We define 54 macros for the powers of two >= 1024.  We use six, in six
macro definitions.  Four of them could just as well use the common MiB
macro, so do that.  The remaining two can't, because they get passed
to stringify.  Replace the macro by the literal number there.
Slightly harder to read in one instance (1048576 vs. S_1MiB), so add a
comment there.  The other instance is a wash: 65536 vs S_64KiB.  65536
has been good enough for more than seven years there.

This effectively reverts commit 540b8492618 and 1240ac558d3.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>