usercopy: Do not select BUG with HARDENED_USERCOPY

There is no need to "select BUG" when CONFIG_HARDENED_USERCOPY is enabled.
The kernel thread will always die, regardless of the CONFIG_BUG.

Signed-off-by: Kamal Mostafa <kamal@canonical.com>
[kees: tweak commit log]
Signed-off-by: Kees Cook <keescook@chromium.org>