netfilter: nf_tables: reject hook configuration updates on existing chains

Currently, if you add a base chain whose name clashes with an existing
non-base chain, nf_tables doesn't complain about this. Similarly, if you
update the chain type, the hook number and priority.

With this patch, nf_tables bails out in case any of this unsupported
operations occur by returning EBUSY.

 # nft add table x
 # nft add chain x y
 # nft add chain x y { type nat hook input priority 0\; }
 <cmdline>:1:1-49: Error: Could not process rule: Device or resource busy
 add chain x y { type nat hook input priority 0; }
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>