If we detect an overlap, we set a flag and wait for a wakeup.  When requests
are handled, if the flag was set, we perform the wakeup.

Note that the code currently in -mm is badly broken.  With this patch applied,
it passes tests the use O_DIRECT to cause lots of overlapping requests.
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>