um: Do not propagate dtb parameter to kernel

This parameter is UML specific and is unknown to kernel. It should not
be propagated to kernel, otherwise it will be passed to user space as
an environment option by kernel with a warning like:

Unknown kernel command line parameters "dtb=/foo", will be passed to user space.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20241011040441.1586345-5-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>