Add possibility for requests to run asynchronously and call an 'end' callback
when finished.

With this, the special handling of the INIT and RELEASE requests can be
cleaned up too.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>