gnttab: limit mapcount() looping

gnttab: limit mapcount() looping

The function doesn't need to return counts in the first place; all its
callers are after is whether at least one entry of a certain kind
exists. With that there's no point for that loop to continue once the
looked for condition was found to be met by one entry. Rename the
function to match the changed behavior.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>