Move rcuwait struct definition into types.h so that rcuwait can be used
without including rcuwait.h which includes other headers. Without this
change mm_types.h can't use rcuwait due to a the following circular
dependency:
mm_types.h -> rcuwait.h -> signal.h -> mm_types.h
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>