ftrace: Add helper function ftrace_hash_move_and_update_ops()

The processes of updating a ops filter_hash is a bit complex, and requires
setting up an old hash to perform the update. This is done exactly the same
in two locations for the same reasons. Create a helper function that does it
in one place.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>