fuse: invalidate the correct range after O_APPEND direct write

In the Linux kernel, the following vulnerability has been resolved:

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").