make domain_adjust_tot_pages() __must_check

make domain_adjust_tot_pages() __must_check

Even if unlikely, donate_page() should not ignore the possible need to
obtain a domain reference. To make people look more closely when they
add new uses of domain_adjust_tot_pages(), force its return value to be
checked. This in turn requires a benign change to assign_pages().

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