uml: reconst a parameter

The previous const-ing patch consted a string which shouldn't have
been, and I didn't notice the gcc warning.

ubd_setup can't take a const char * because its address is assigned to
something which expects a char *arg.  Many setups modify the string
they are given, they can't be const.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>