xen/x86: mem_sharing: Use copy_domain_page in __mem_sharing_unshare_page

xen/x86: mem_sharing: Use copy_domain_page in __mem_sharing_unshare_page

The function __mem_sharing_unshare_page contains an open-code version of
copy_domain_page. Use the function to simplify a bit the code.

At the same time replace _mfn(__page_to_mfn(...)) by page_to_mfn(...)
given that the file given already provides a typesafe version of page_to_mfn.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>