xen/privcmd: allow fetching resource sizes

commit ef3a575baf53571dc405ee4028e26f50856898e7 upstream.

Allow issuing an IOCTL_PRIVCMD_MMAP_RESOURCE ioctl with num = 0 and
addr = 0 in order to fetch the size of a specific resource.

Add a shortcut to the default map resource path, since fetching the
size requires no address to be passed in, and thus no VMA to setup.

This is missing from the initial implementation, and causes issues
when mapping resources that don't have fixed or known sizes.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: stable@vger.kernel.org # >= 4.18
Link: https://lore.kernel.org/r/20210112115358.23346-1-roger.pau@citrix.com


Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>