netfilter: nf_tables: report EEXIST on overlaps

Replace EBUSY by EEXIST in the following cases:

- If the user adds a chain with a different configuration such as different
  type, hook and priority.

- If the user adds a non-base chain that clashes with an existing basechain.

- If the user adds a { key : value } mapping element and the key exists
  but the value differs.

- If the device already belongs to an existing flowtable.

User describe that this error reporting is confusing:

- https://bugzilla.netfilter.org/show_bug.cgi?id=1176
- https://bugzilla.netfilter.org/show_bug.cgi?id=1413



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