fuse: invalidate the correct range after O_APPEND direct write

fuse_direct_write_iter() captures pos before generic_write_checks(),
which moves ki_pos to EOF for O_APPEND writes:

  fuse_direct_write_iter()
  {
      pos = iocb->ki_pos;           /* 0 (user-supplied)       */
      generic_write_checks();       /* ki_pos -> EOF           */
      fuse_direct_io();             /* writes at EOF, correct  */
      invalidate(pos, pos + res);   /* [0, res) -- wrong       */
  }

The post-write invalidation targets a stale range instead of the
actual written range at EOF.

This can cause data inconsistency when the file size is not
page-aligned.  The tail page straddling EOF has a valid portion
before EOF that concurrent readers can fault back in during the
DIO write window:

  Tail page (file size X not page-aligned):

    page_start         X (EOF)   page_end
    |--- valid data ----|-- stale --|

  CPU0 (O_APPEND DIO writer)    CPU1 (buffered reader)
  --------------------------    ----------------------
  invalidate [X, X+len)
    tail page evicted
  FUSE_WRITE in flight ...
                                read [page_start, X)
                                  tail page re-faulted
                                  [X, page_end) = stale
  FUSE_WRITE completes
  i_size = X + len
  invalidate [0, len)  <- WRONG
    tail page still cached
                                read [X, X+len)
                                  hits stale tail page
                                  returns old data

Fix by reading pos back from iocb->ki_pos after generic_write_checks(),
as generic_file_direct_write() does.

Also fix a typo in the comment ("may have" -> "may have competed").

Fixes: 2b0408d0284f ("fuse: invalidate page cache after DIO and async DIO writes")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Reviewed-by: Bernd Schubert <bernd@bsbernd.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>