zdtm: Make kernel version check more POSIX-like

Arrays and "&&" in test are not supported by Busybox v1.19.4. The
`set` command can put field-separated arguments into the positional
parameters, which is relatively equivalent to using an array.

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>