rmmod: fix bad error message

rmmod: fix bad error message

    Before:

    ># busybox_old rmmod gtrhfhdfghdf
    rmmod: can't unload 'gtrhfhdfghdf': unknown symbol in module, or unknown parameter

    After:

    ># busybox rmmod gtrhfhdfghdf
    rmmod: can't unload module 'gtrhfhdfghdf': No such file or directory

function                                             old     new   delta
modprobe_main                                        726     721      -5
do_modprobe                                          599     590      -9
rmmod_main                                           187     169     -18
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-32)             Total: -32 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>