ubifs: Fix WARN_ON logic in exit path

ubifs_assert() is not WARN_ON(), so we have to invert
the checks.
Randy faced this warning with UBIFS being a module, since
most users use UBIFS as builtin because UBIFS is the rootfs
nobody noticed so far. :-(
Including me.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: 54169ddd ("ubifs: Turn two ubifs_assert() into a WARN_ON()")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>