devlink: Allow setting of packet trap group parameters

The previous patch allowed device drivers to publish their default
binding between packet trap policers and packet trap groups. However,
some users might not be content with this binding and would like to
change it.

In case user space passed a packet trap policer identifier when setting
a packet trap group, invoke the appropriate device driver callback and
pass the new policer identifier.

v2:
* Check for presence of 'DEVLINK_ATTR_TRAP_POLICER_ID' in
  devlink_trap_group_set() and bail if not present
* Add extack error message in case trap group was partially modified

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>