use return value of domain_adjust_tot_pages() where feasible

use return value of domain_adjust_tot_pages() where feasible

This is generally cheaper than re-reading ->tot_pages.

While doing so I also noticed an improper use (lacking error handling)
of get_domain() as well as lacks of ->is_dying checks in the memory
sharing code, which the patch fixes at once. In the course of doing
this I further noticed other error paths there pointlessly calling
put_page() et al with ->page_alloc_lock still held, which is also being
reversed.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: Keir Fraser <keir@xen.org>