netfilter: nf_tables: rise cap on SELinux secmark context

[ Upstream commit e29630247be24c3987e2b048f8e152771b32d38b ]

secmark context is artificially limited 256 bytes, rise it to 4Kbytes.

Fixes: fb961945 ("netfilter: nf_tables: add SECMARK support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>