zstd: Fix definition of assert()

assert(x) should emit a warning if x is false. WARN_ON(x) emits a
warning if x is true. Thus, assert(x) should be defined as WARN_ON(!x)
rather than WARN_ON(x).

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Nick Terrell <terrelln@fb.com>