From: Suparna Bhattacharya <suparna@in.ibm.com>

Fixes the following remaining issues with the DIO code:

1. During DIO file extends, intermediate writes could extend i_size
   exposing unwritten blocks to intermediate reads (Soln: Don't drop i_sem
   for file extends)

2. AIO-DIO file extends may update i_size before I/O completes,
   exposing unwritten blocks to intermediate reads.  (Soln: Force AIO-DIO
   file extends to be synchronous)

3. AIO-DIO writes to holes call aio_complete() before falling back to
   buffered I/O !  (Soln: Avoid calling aio_complete() if -ENOTBLK)

4. AIO-DIO writes to an allocated region followed by a hole, falls back
   to buffered i/o without waiting for already submitted i/o to complete;
   might return to user-space, which could overwrite the buffer contents
   while they are still being written out by the kernel (Soln: Always wait
   for submitted i/o to complete before falling back to buffered i/o)