crypto: zstd - Fix compression bug caused by truncation

Use size_t for the return value of zstd_compress_cctx as otherwise
negative errors will be truncated to a positive value.

Reported-by: Han Xu <han.xu@nxp.com>
Fixes: f5ad93ff ("crypto: zstd - convert to acomp")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: David Sterba <dsterba@suse.com>
Tested-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>