mountd: change "unknown host" error message to "unmatched host"

mountd: change "unknown host" error message to "unmatched host"

mount request from unknown host 10.11.14.99 for /export

The hosts are listed in DNS with proper reverse records, so the reason
why the host is "unknown" isn't clear. This patch just changes the
wording of this error to hopefully make it more clear why the mount
request was rejected. This also makes this error message use a format
more similar to the other error messages in auth_authenticate().

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>