Part of NBD's 64-bit headers extension involves passing the client's
requested offset back as part of the reply header (one reason for this
change: converting absolute offsets stored in
NBD_REPLY_TYPE_OFFSET_DATA to relative offsets within the buffer is
easier if the absolute offset of the buffer is also available). This
is a refactoring patch to pass the full request around the reply
stack, rather than just the handle, so that later patches can then
access request->from when extended headers are active. But for this
patch, there are no semantic changes.
Signed-off-by: Eric Blake<eblake@redhat.com>