Under certain conditions, the range to be cleared by FALLOC_FL_ZERO_RANGE
may only be buffered locally and not yet have been flushed to the server.
For example:

	xfs_io -f -t -c "pwrite -S 0x41 0 4k" \
		     -c "pwrite -S 0x42 4k 4k" \
		     -c "fzero 0 4k" \
		     -c "pread -v 0 8k" /xfstest.test/foo

will write two 4KiB blocks of data, which get buffered in the pagecache,
and then fallocate() is used to clear the first 4KiB block on the server -
but we don't flush the data first, which means the EOF position on the
server is wrong, and so the FSCTL_SET_ZERO_DATA RPC fails (and xfs_io
ignores the error), but then when we try to read it, we see the old data.

Fix this by preflushing any part of the target region that above the
server's idea of the EOF position to force the server to update its EOF
position.

Note, however, that we don't want to simply expand the file by moving the
EOF before doing the FSCTL_SET_ZERO_DATA[*] because someone else might see
the zeroed region or if the RPC fails we then have to try to clean it up or
risk getting corruption.

[*] And we have to move the EOF first otherwise FSCTL_SET_ZERO_DATA won't
do what we want.

This fixes the generic/008 xfstest.

[!] Note: A better way to do this might be to split the operation into two
parts: we only do FSCTL_SET_ZERO_DATA for the part of the range below the
server's EOF and then, if that worked, invalidate the buffered pages for the
part above the range.

Fixes: 6b690402 ("cifs/smb3: Fix data inconsistent when zero file range")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <stfrench@microsoft.com>
cc: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
cc: Pavel Shilovsky <pshilov@microsoft.com>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Steve French <stfrench@microsoft.com>