ftrace: Use rcu_assign_pointer() for tmp_ops filter hash

tmp_ops.func_hash->filter_hash is annotated __rcu, but
update_ftrace_direct_mod() assigns hash to it directly. Sparse reports an
address-space mismatch.

Use rcu_assign_pointer() for the assignment.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260911142512.19344-1-leon.hwang@linux.dev
Fixes: 50b35c9e50a8 ("ftrace: Use hash argument for tmp_ops in update_ftrace_direct_mod")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202609110704.Q3M5vCDV-lkp@intel.com/
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>