ovl: relax WARN_ON in ovl_verify_area()

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