netfilter: nf_tables: replace BUG_ON by element length check

BUG_ON can be triggered from userspace with an element with a large
userdata area. Replace it by length check and return EINVAL instead.
Over time extensions have been growing in size.

Pick a sufficiently old Fixes: tag to propagate this fix.

Fixes: 7d740264 ("netfilter: nf_tables: variable sized set element keys / data")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>