xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored

xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored

Or if it's not possible to honor the hinted address an error is returned
instead. This makes it easier to spot the actual failure, instead of
failing later on when the caller of xen_remap_bucket realizes the
mapping has not been created at the requested address.

Also note that at least on FreeBSD using MAP_FIXED will cause mmap to
try harder to honor the passed address.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Igor Druzhinin <igor.druzhinin@cirtix.com>
Message-Id: <20190318173731.14494-1-roger.pau@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>