netlink: Don't use a zero-length array

Define SNL_DECLARE_FIELD_PARSER* macros to create a parser that has
no output attributes only input fields and use this to define the
snl_donemsg_parser.

This removes the need for the zero-length nla_p_donemsg[] variable.
Zero length arrays are not valid in ISO C.

Reviewed by: jrtc27, melifaro
Differential Revision: https://reviews.freebsd.org/D43918