Jump label reworked

There were a number of complaints about the initial jump label implementation, including the fact that it was somewhat awkward to use. In response, a reworked version has been posted which changes the interface considerably. One starts by declaring a "jump key":

    #include <linux/jump_label.h>

    struct jump_label_key my_key;

Enabling and disabling the key is a simple matter of calling:

    jump_label_inc(struct jump_label_key *key);
    jump_label_dec(struct jump_label_key *key);

And using the key to control the execution of rarely-needed code becomes:

    if (static_branch(&my_key)) {
	/* Unlikely stuff happens here */
    }

In the absence of full jump label support, a jump key is represented by an atomic_t value. jump_label_inc() becomes atomic_inc(), jump_label_dec() becomes atomic_dec(), and static_branch() is implemented with atomic_read(). If jump label is configured into the kernel, enabling and disabling a jump key become heavier operations, while static_branch() becomes nearly free. For the intended use cases for jump labels, that is a worthwhile tradeoff.

As of this writing, these changes have not been merged for 2.6.39. There is always a possibility that they could be pulled in before -rc2, but chances are that, at this point, the new jump label will have to jump into 2.6.40.