kernel: make get_image_version() work for rpm

kernel: make get_image_version() work for rpm

At some point in the past, the layout of the rpm package has changed.
There is no file matching "/boot/vmlinuz-" there anymore, instead there
is "vmlinuz" file at the root of the modules directory.  For reference:

deb:

-rw-r--r-- root/root  11527168 2024-02-13 16:25 ./boot/vmlinuz-6.8.0-rc4-ga64ccd305b28
-rw-r--r-- root/root     72614 2024-02-13 16:25 ./lib/modules/6.8.0-rc4-ga64ccd305b28/modules.order

rpm:

/lib/modules/6.8.0-rc4-ga64ccd305b28/modules.order
/lib/modules/6.8.0-rc4-ga64ccd305b28/vmlinuz

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>