netfilter: nf_tables: set element extended ACK reporting support

commit b53c116642502b0c85ecef78bff4f826a7dd4145 upstream.

Report the element that causes problems via netlink extended ACK for set
element commands.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 61fbbac22c8ce73d0c492caf45a286c3f021c0fd)
[Vegard: fix conflict in nf_tables_getsetelem() due to missing commit
 ba0e4d9917b43dfa746cbbcb4477da59aae73bd6 ("netfilter: nf_tables: get
 set elements via netlink") from v4.15]
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>