tests/9p: fix Rreaddir response name

tests/9p: fix Rreaddir response name

All 9p response types are prefixed with an "R", therefore fix
"READDIR" -> "RREADDIR" in function rmessage_name().

Fixes: 4829469fd9ff ("tests/virtio-9p: added readdir test")
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <daad7af58b403aaa2487c566032beca36664b30e.1732465720.git.qemu_oss@crudebyte.com>
(cherry picked from commit abf0f092c1dd33b9ffa986c6924addc0a9c1d0b8)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>