expr: fix crash when listing non-verdict mappings

expr: fix crash when listing non-verdict mappings

Fix regression introduced by commit 87c2a2205:

  netlink_delinearize: clone on netlink_get_register(), release previous on _set()

When using a non-verdict mapping, the set ref expression is assigned to the
destination register. The next get_register() will attempt to clone it and
crash because of the missing ->clone() callback.

# nft filter input meta mark set ip daddr map { 192.168.0.1 : 123 }
# nft list table filter
Segmentation fault (core dumped)

Signed-off-by: Patrick McHardy <kaber@trash.net>