x86: correct instances of PGC_allocated clearing

x86: correct instances of PGC_allocated clearing

For domain heap pages assigned to a domain dropping the page reference
tied to PGC_allocated may not drop the last reference, as otherwise the
test_and_clear_bit() might already act on an unowned page.

Work around this where possible, but the need to acquire extra page
references is a fair hint that references should have been acquired in
other places instead.

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