ovl: relax WARN_ON in ovl_verify_area()

syzbot hit an assertion in copy up data loop which looks like it is
the result of a lower file whose size is being changed underneath
overlayfs.

This type of use case is documented to cause undefined behavior, so
returning EIO error for the copy up makes sense, but it should not be
causing a WARN_ON assertion.

Reported-and-tested-by:  <syzbot+3abd99031b42acf367ef@syzkaller.appspotmail.com>
Fixes: ca7ab482 ("ovl: add permission hooks outside of do_splice_direct()")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>