um: Use err consistently in userspace()

Avoid declaring a new variable 'ret' inside the 'if (using_seccomp)'
block, as the existing 'err' variable declared at the top of the
function already serves the same purpose.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250711065021.2535362-2-tiwei.bie@linux.dev


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