xenbus: maximum buffer size is XENSTORE_PAYLOAD_MAX

Use this now that it is defined even though it happens to be == PAGE_SIZE.

The code which takes requests from userspace already validates against the size
of this buffer so no further checks are required to ensure that userspace
requests comply with the protocol in this respect.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Haogang Chen <haogangchen@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>