Use interruptible sleep rather than uninterruptible.

Partially convert it to the kthread API so the kernel thread doesn't get
accidentally signalled.
Signed-off-by: Colin Leroy <colin@colino.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>