blkif: drop struct blkif_request_segment_aligned

blkif: drop struct blkif_request_segment_aligned

Commit 5148b7b5 ("blkif: add indirect descriptors interface to public
headers") added this without really explaining why it is needed: The
structure is identical to struct blkif_request_segment apart from the
padding field not being given a name in the pre-existing type. Their
size and alignment - which are what is relevant - are identical as long
as __alignof__(uint32_t) == 4 (which I think we rely upon in various
other places, so we can take as given).

Also correct a few minor glitches in the description, including for it
to no longer assume PAGE_SIZE == 4096.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>