livepatch: Convert error about unsupported reliable stacktrace into a warning

The commit d0807da7 ("livepatch: Remove immediate feature") caused
that any livepatch was refused when reliable stacktraces were not supported
on the given architecture.

The limitation is too strong. User space processes are safely migrated
even when entering or leaving the kernel. Kthreads transition would
need to get forced. But it is safe when:

   + The livepatch does not change the semantic of the code.
   + Callbacks do not depend on a safely finished transition.
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/livepatch/core.c

@@ -1003,11 +1003,10 @@ int klp_enable_patch(struct klp_patch *patch)
 		return -ENODEV;
 
 	if (!klp_have_reliable_stack()) {
-		pr_err("This architecture doesn't have support for the livepatch consistency model.\n");
-		return -EOPNOTSUPP;
+		pr_warn("This architecture doesn't have support for the livepatch consistency model.\n");
+		pr_warn("The livepatch transition may never complete.\n");
 	}
 
-
 	mutex_lock(&klp_mutex);
 
 	ret = klp_init_patch_early(patch);