configs/debug: run and debug PREEMPT

Recent change [0] resulted in a "BUG: using __this_cpu_read() in
preemptible" splat [1]. PREEMPT kernels have additional requirements
on what can and can not run with/without preemption enabled.
Expose those constrains in the debug kernels.

0: https://lore.kernel.org/netdev/20250314120048.12569-2-justin.iurman@uliege.be/
1: https://lore.kernel.org/netdev/20250402094458.006ba2a7@kernel.org/T/#mbf72641e9d7d274daee9003ef5edf6833201f1bc

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250402172305.1775226-1-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Stanislav Fomichev

kernel/configs/debug.config

@@ -112,3 +112,8 @@ CONFIG_BRANCH_PROFILE_NONE=y
 CONFIG_DYNAMIC_FTRACE=y
 CONFIG_FTRACE=y
 CONFIG_FUNCTION_TRACER=y
+#
+# Preemption
+#
+CONFIG_DEBUG_PREEMPT=y
+CONFIG_PREEMPT=y