mnl: flowtable support for extended netlink error reporting

mnl: flowtable support for extended netlink error reporting

commit 99b56d4ee1442ccfa0aadde3cb3ecee74a4de815 upstream.

This patch extends existing flowtable support to improve error
reporting:

 # nft add flowtable inet x y '{ devices = { x } ; }'
 Error: Could not process rule: No such file or directory
 add flowtable inet x y { devices = { x } ; }
                                      ^
 # nft delete flowtable inet x y '{ devices = { x } ; }'
 Error: Could not process rule: No such file or directory
 delete flowtable inet x y { devices = { x } ; }
                                         ^
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>