cifs: don't try to unlock pagecache page after releasing it

We had a recent fix to fix the release of pagecache pages when
cifs_writev_requeue writes fail. Unfortunately, it releases the page
before trying to unlock it. At that point, the page might be gone by the
time the unlock comes in.

Unlock the page first before checking the value of "rc", and only then
end writeback and release the pages. The page lock isn't required for
any of those operations so this should be safe.

Reported-by: Anton Altaparmakov <aia21@cam.ac.uk>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>