trace: fix an incorrect __user annotation on stack_trace_sysctl

No user pointers for sysctls anymore.

Fixes: 32927393 ("sysctl: pass kernel pointers to ->proc_handler")
Reported-by: build test robot <lkp@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>