re-enable IOCB_NOWAIT writes to files v4

[Christoph Hellwig]

> Applied to the vfs-7.0.nonblocking_timestamps branch of the vfs/vfs.git tree.
> Patches in the vfs-7.0.nonblocking_timestamps branch should appear in linux-next soon.

Umm, as in my self reply just before heading out for vacation, Julia
found issues in it using static type checking tools.  I have a new
version that fixes that and sorts out the S_* mess.  So please drop
it again for now, I'll resend the fixed version ASAP.