netfilter: nft_compat: narrow down revision to unsigned 8-bits

[ Upstream commit 36fa8d69 ]

xt_find_revision() expects u8, restrict it to this datatype.

Fixes: 0ca743a5

 ("netfilter: nf_tables: add compatibility layer for x_tables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>