xen/gnttab: Reduce code volume when using union grant_combo

xen/gnttab: Reduce code volume when using union grant_combo

There is no need for 'struct { ... } shorts' to be named.  Convert it to being
an anonymous struct, and rename 'word' to the more common 'raw'.

For _set_status_v1() and gnttab_prepare_for_transfer() which use a bounded
cmpxchg loop, rename {prev,new}_scombo to {prev,new} and reduce their scope to
within the loop.

For _set_status_v2(), the flags and id variables are completely unnecessary.
Drop them.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>