x86/retpoline: Fix crash printing warning

The first argument of WARN() is a condition, so this will use "addr"
as the format string and possibly crash.

Fixes: 3b6c1747 ("x86/retpoline: Add SKL retthunk retpolines")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/Y1gBoUZrRK5N%2FlCB@kili/