crypto: scompress - Fix scratch allocation failure handling

If the scratch allocation fails, all subsequent allocations will
silently succeed without actually allocating anything.  Fix this
by only incrementing users when the allocation succeeds.

Fixes: 6a8487a1 ("crypto: scompress - defer allocation of scratch buffer to first use")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>