kmod: make usermodehelper path a const string

This is in preparation for making it so that usermode helper programs
can't be changed, if desired, by userspace.  We will tackle the mess of
cleaning up the write-ability of argv and env later, that's going to
take more work, for much less gain...

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>