standardized waitqueue hashing

The following patch series consolidates the various instances of waitqueue hashing to use a uniform structure and share the per-zone hashtable among all waitqueue hashers. This is expected to increase the number of hashtable buckets available for waiting on bh's and inodes and eliminate statically allocated kernel data structures for greater node locality and reduced kernel image size. Some attempt was made to look similar to Oleg Nesterov's suggested API in order to provide some kind of credit for independent invention of something very similar (the original versions of these patches predated my public postings on the subject of filtered waitqueues).

These patches have the further benefit and intention of enabling aio to use filtered wakeups by standardizing the data structure passed to wake functions so that embedded waitqueue elements in aio structures may be succesfully passed to the filtered wakeup wake functions, though this patch series doesn't implement that particular functionality.

Successfully stress-tested on x86-64, and ia64 in recent prior versions.

-- wli