mm.h: add helper function to test-and-clear _PGC_allocated

mm.h: add helper function to test-and-clear _PGC_allocated

The _PGC_allocated flag is set on a page when it is assigned to a domain
along with an initial reference count of at least 1. To clear this
'allocation' reference it is necessary to test-and-clear _PGC_allocated and
then only drop the reference if the test-and-clear succeeds. This is open-
coded in many places. It is also unsafe to test-and-clear _PGC_allocated
unless the caller holds an additional reference.

This patch adds a helper function, put_page_alloc_ref(), to replace all the
open-coded test-and-clear/put_page occurrences. That helper function
incorporates a check that an additional page reference is held and will
BUG() if it is not.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>