The last fix for this function in fact opened up a much more often
triggering race.

It was uncommented tricky code, that was buggy.  Add comment, make it less
tricky and fix bug.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>