cifs: Fix cifs_writepages_region()

Fix the cifs_writepages_region() to just jump over members of the batch
that have been cleaned up rather than counting them as skipped.

Unlike the other "skip_write" cases, this situation happens even for
WB_SYNC_ALL, simply because the page has either been cleaned by somebody
else, or was truncated.

So in this case we're not "skipping" the write, we simply no longer need
any write at all, so it's very different from the other skip_write cases.

And we definitely shouldn't stop writing the rest just because of too
many of these cases (or because we want to be rescheduled).

Fixes: 3822a7c40997 ("Merge tag 'mm-stable-2023-02-20-13-37' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/lkml/2213409.1677249075@warthog.procyon.org.uk/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>