xen/arm: introduce put_page_nr and get_page_nr

xen/arm: introduce put_page_nr and get_page_nr

Later, we need to add the right amount of references, which should be
the number of borrower domains, to the owner domain. Since we only have
get_page() to increment the page reference by 1, a loop is needed per
page, which is inefficient and time-consuming.

To save the loop time, this commit introduces a set of new helpers
put_page_nr() and get_page_nr() to increment/drop the page reference by nr.

Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Julien Grall <jgrall@amazon.com>