netfilter: nft_compat: fix error path in nft_parse_compat()

The patch 0ca743a5

: "netfilter: nf_tables: add compatibility
layer for x_tables", leads to the following Smatch

 warning: "net/netfilter/nft_compat.c:140 nft_parse_compat()
          warn: signedness bug returning '(-34)'"

This nft_parse_compat function returns error codes but the return
type is u8 so the error codes are transformed into small positive
values. The callers don't check the return.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>